Closes #15

- change Gitlab-CI Docker image to latest LHCb one
- prefer host-binary-tag from PATH
  (see https://gitlab.cern.ch/lhcb-core/LbPlatformUtils/)
- use xenv from PATH or get it from the Git repository
  (see https://gitlab.cern.ch/gaudi/xenv)

if taken from Git it's also distributed in the installation directory

otherwise fall back on the provided get_host_binary_tag.py

partial overlap with !574

add target `test_headers_build` to test build of headers

- Replace intrusive reference counting of Tuples with std::shared_ptr   
- Use a single boost multi-index map to keep track of Tuples
- prefer invoke, avoid std::function if possible, when applying 'callables'
  to objects.

- require public constructor for components
- removed XyzFactory classes
- enabled warning for XyzFactory headers
- some clean up and modernization

deReg deleted the object it is deRegistering, but the main purpose of
this method is to undo the regiser process, which essentially means to
just give up ownership without deletion!

Use thread-local copies of the ToolHandleArray

If a property contains `[[deprecated]]` in its doc string, print a
deprecation warning on setting the property value.

Fixes gcc8 compilation error with

prop["foo"]

where prop is a Property<std::map<std::string, std::string> >.

One gets an error like this (trimmed for brevity):

Control/AthenaExamples/AthExHelloWorld/src/HelloAlg.cxx: In constructor 'HelloAlg::HelloAlg(const string&, ISvcLocator*)':
Control/AthenaExamples/AthExHelloWorld/src/HelloAlg.cxx:26:11: error: no match for 'operator[]' (operand types are 'Gaudi::Property<std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > >' and 'const char [8]')
   m_myDict["Bonjour"]      = "Guten Tag";
           ^
In file included from /home/sss/atlas/extern/Gaudi/current/GaudiKernel/GaudiKernel/GaudiHandle.h:7,
                 from /home/sss/atlas/extern/Gaudi/current/GaudiKernel/GaudiKernel/ToolHandle.h:5,
                 from Control/AthenaExamples/AthExHelloWorld/src/HelloAlg.h:10,
                 from Control/AthenaExamples/AthExHelloWorld/src/HelloAlg.cxx:9:
/home/sss/atlas/extern/Gaudi/current/GaudiKernel/GaudiKernel/Property.h:544:49: note: candidate: 'template<class ARG, class T> decltype (declval<T>()[ARG{}]) Gaudi::Property<TYPE, VERIFIER, HANDLERS>::operator[](const ARG&) [with ARG = ARG; T = T; TYPE = std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >; VERIFIER = Gaudi::Details::Property::NullVerifier; HANDLERS = Gaudi::Details::Property::UpdateHandler]'
     inline decltype( std::declval<T>()[ARG{}] ) operator[]( const ARG& arg )
                                                 ^~~~~~~~
/home/sss/atlas/extern/Gaudi/current/GaudiKernel/GaudiKernel/Property.h:544:49: note:   template argument deduction/substitution failed:
/home/sss/atlas/extern/Gaudi/current/GaudiKernel/GaudiKernel/Property.h: In substitution of 'template<class ARG, class T> decltype (declval<T>()[ARG{}]) Gaudi::Property<std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > >::operator[]<ARG, T>(const ARG&) [with ARG = char [8]; T = std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >]':
Control/AthenaExamples/AthExHelloWorld/src/HelloAlg.cxx:26:21:   required from here
/home/sss/atlas/extern/Gaudi/current/GaudiKernel/GaudiKernel/Property.h:544:39: error: taking address of temporary array
     inline decltype( std::declval<T>()[ARG{}] ) operator[]( const ARG& arg )
                      ~~~~~~~~~~~~~~~~~^


Easiest fix seems to be to use decltype(auto) for the return types.

If a property contains '[[deprecated]]' in its doc string, print a
deprecation warning on setting the property value.

ToolHandle is not thread-safe.
In order to be able to run thread initialization in parallel,
we need to make thread-local copies of the ToolHandleArray.

Needed to allow running part of the G4 initialization in parallel
for the ATLAS simulation.  Makes a significant difference  on many-core
machines like KNL.