Modernize/cleanup UT code

• simplify interaction between stripflip() and xInverted()
• add const where appropriate
• prefer C++ headers over C headers (eg. cstddef instead of stddef.h)
• prefer std::memcpy over assignment to a reinterpret_cast-ed object (alignment!)
• prefer inherited constructors instead of explictly forwarding constructructors
• prefer string_view over const std::string&
• prefer pragma once over header guards
• make sure order of include files does not matter
• let clang-format order the include files
• remove unneccessary mutable qualifiers
• avoid unneccessary copies
• prefer standalone functions in anonymous namespace over member functions
• remove unneccessary destructors
• prefer boost::flat_map over GaudiUtils::VectorMap as it supports transparant lookup
• prefer charconv and error check over stoul and exception handling, as it also allows the use of string_view
• prefer member initialization on declaration
• define trivial inline functions at their point of declaration
• prefer monadic StatusCode functionality
• do not compare bool to literal true or false in conditionals
• move operator<< out of global namespace, make friend instead, change argument type from pointer to object
• leave [[deprecated]] version in global namespace which takes pointer
• move use of UTNames() out of UTDigit.h into UTDigit.cpp to keep it away from cling, which doesn't like boost::flat_map
• update CMakeLists to reflect that DigitEvent now has a (tiny) library
• remove UTNames and UTCluster from dictionary

must be applied in conjunction with Lbcom!538 (merged), Boole!328 (merged), Rec!2323 (merged)