- Add Gaudi::apply -- an implementation of C++17 std::apply
- Prefer Gaudi::apply over direct use of std::index_sequence:
    - eliminates the explicit 'two-step' call through an `invoke` member
      function required to generate an `std::index_sequence`
    - enables RVO in multi transformer case

See merge request !622

- eliminates the explicit 'two-step' call through an `invoke` member
  function required to generate an `std::index_sequence`
- enables RVO in multi transformer case

- fix "dereference after null check" error introduced with e6f3bc22 (mr
!455)
- fix "uninitialized member" error introduced with 99edb6c4 (mr !420)

https://coverity.cern.ch/reports.htm#v13572/p10041/fileInstanceId=763878155&defectInstanceId=396360325&mergedDefectId=98200

https://coverity.cern.ch/reports.htm#v13572/p10041/fileInstanceId=763879735&defectInstanceId=396360581&mergedDefectId=98201

Change

operator<<( std::ostream& s, AlgsExecutionStates::State x )

to convert to string instead of integer. The latter was nowhere used.
Use the operator in EntityState to avoid the duplicate enum-to-string
hard-coding.

This reverts merge request !636

and add a test which checks the ordering of EventIDBase

* 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.