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.

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.

Coverity claims:

 error #2443: defaulted default constructor cannot be constexpr
          because the corresponding implicitly declared default constructor
          would not be constexpr
    constexpr StatusCode() = default;

Independent of that there is little use of declaring the default
constructor as constexpr as it is calling the non-constexpr
default_category().

The lambda in deReg( TObject* ) tried to access vhid->at(0) after a
check for vhid->size() == 0. This resulted in a certain crash.

I've reorganized the function to delete the vector of THistIDs when
vhid->size() is 1 (not 0).
There is also a new test case that registers and deregisters a single
histogram.

Apply formatting

Add legacy methods for regGraph and regTree

Migrating the Athena in these cases would be possible, but not very
clean. This is why I decided to favor a consistent interface (with
similar methods for Hists, Trees and Graphs), opposed to only providing
unique_ptr methods. It would be good if future THistSvc clients would
pick up the new methods and old methods would be migrated over time.