Merge branch '722-add-code-style-naming-guidelines' into 'master'

Add code style/ naming guidelines

Closes #722

See merge request !796

doc/codeguide.md

+# Coding guidelines
+
+The following guidelines must be followed by all new code. Existing code that does not yet follow should be adapted when possible.
+You might disagree with some guidelines, but in a large code base as this one consistency is more important than personal opinion.
+All guidelines have a short identifier label, e.g. N.1, for easier reference in discussions.
+
+For cases and constructs not explicitely mentioned here, code should fall back to the [C++ Core Guidelines](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines).
+
+## Naming
+
+### N.1: Namespaces use CamelCase
+
+Namespaces use CamelCase with an upper case initial.
+
+Example:
+
+```cpp
+namespace Something {
+...
+}
+```
+
+The only exception is the nested `detail` namespace with always uses the lower-case name. There is no particular strong reason but existing practice for this. The `detail` namespace must be never be the top-most namespace.
+
+Example:
+
+```cpp
+namespace Foo {
+namespace detail {
+...
+}
+}
+```
+
+### N.2: Concrete types use CamelCase
+
+All concrete types, i.e. classes, structs, enums, and typedefs for both fundamental and external types, use CamelCase with an upper case initial.
+
+Example:
+
+```cpp
+class ComplicatedNamedClass;
+struct AnotherStruct;
+enum EnumType;
+using SemanticName = int32;
+```
+
+Prefer semantic names over descriptive names. This applies in particular to typedefs of external types or standard containers. Types are checked by the compiler, while semantic meaning is only checked by the user; optimize for the latter.
+
+Example:
+
+```cpp
+// avoid: purely descriptive typename
+using ConstThingPtrVector = std::vector<const Thing*>;
+// prefer: semantic typename the describes the usage
+using InputThings = std::vector<const Thing*>;
+```
+
+Avoid abbreviations and prefer full names. If abbreviations must be used, only use upper case for the initial letter of the abbreviation. The abbreviation is semantically a single word and this provides better visual consistency with the CamelCase naming.
+
+Example:
+
+```cpp
+// VertexSomething should be abbreviated as
+struct VtxSomething; // not VTXSomething.
+// This is bad example since the name should not be abbreviated here anyways.
+```
+
+### N.3: Functions and methods use mixedCase
+
+All functions and methods use mixedCase with a lower case initial. This applies also to private helper functions, e.g. in the `detail` namespace. These are not fundamentally different from other functions and do not warrant a separate convention just by virtue of being e.g. in the `detail` namespace.
+
+Example:
+
+```cpp
+doSomething(...);
+thing.doSomething(...);
+detail::anImplementationDetail(...);
+```
+
+### N.4: Variables use mixedCase with appropriate prefix
+
+Local variables, including function arguments, and public member variables use mixedCase with lower case initial.
+
+Example:
+
+```cpp
+int aVariable;
+struct Thing {
+  double aPublicVariable;
+  float anotherPublicOne;
+};
+```
+
+Private member variables have an additional `m_` prefix.
+
+Example:
+
+```cpp
+class Foo {
+private:
+  m_privateMember;
+};
+```
+
+Static variables, i.e. local, global, and member ones, have an additional `s_` prefix to mark them as the satanic tools that they are.
+
+```cpp
+static int s_thisIsBadAndYouShouldFeelBad = -42;
+class Bar {
+    static double s_notMuchBetter = 3.14;
+};
+```
+
+### N.5: Constant values use kCamelCase
+
+Variables representing constant values use CamelCase with a `k` prefix. Only variables marked as `constexpr` are considered constant values.
+
+Example:
+
+```cpp
+static constexpr double kMagic = 1.23;
+```
+
+Variables defined in the `Acts::UnitConstants` namespace are excempted for usability reasons and use regular variable naming instead.
+ 
+### N.6: Enum values use eCamelCase
+
+Enum values use CamelCase with a `e` prefix. They are not really constants but symbolic values, e.g. they can never have an address, and warant a separate convention.
+
+Example:
+
+```cpp
+enum Bar {
+  eBla,
+  eBlub,
+};
+```
+
+### N.7: Template parameter types use snake_case_t
+
+Template parameter types use lower case words, separated by underscores, with a `_t` suffix. The separate naming convention from concrete types clarifies that these are variable types and avoids naming collisions when reexporting the type.
+
+Example:
+
+```cpp
+template<typename large_vector_t>
+struct Algorithm {
+  // make the template type publicly available
+  using LargeVector = large_vector_t;
+  ...
+};
+```
+
+### N.8: Macros use ACTS_ALL_UPPER_CASE
+
+Do not use macros. You want to use macros? Do not use them! Are you even listening? Ok, you are not: macros use `UPPER_CASE` with underscores as word separators and a mandatory `ACTS_` prefix.
+
+Example:
+
+```
+#define ACTS_I_DID_NOT_LISTEN(...) ...
+```
+
+### N.9: Files use CamelCase
+
+Files use CamelCase with upper case initial. If the file defines a single class/struct, the filename must match the typename. Otherwise, a common name describing the shared intend should be used.
+
+Source files use the `.cpp` extension, Header files use the `.hpp` extension, inline implementation files use the `.ipp` extensions.
+
+Files that contain only symbols in the `detail` namespace should be moved in a`detail/` directory in the module directory. This should not be done for corresponding source files.
+
+## Formatting
+
+Code formatting is handled by [ClangFormat](https://clang.llvm.org/docs/ClangFormat.html). A configuration file is available in the repository root. Please set up your editor/IDE to format files automatically.
+