* mr-639:
  Remove deprecation warning from ExtraInputs and ExtraOutputs properties

* mr-638:
  Fix warnings seen when compiling with gcc8.

* mr-636:
  GaudiProjectConfig: Fix generated __init__ files to handle symlinks.

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!

They cannot be deprecated until there is an alternative method to handle
artificial dependencies.

Use thread-local copies of the ToolHandleArray