Co-authored-by: Andre Sailer <andre.philippe.sailer@cern.ch>

Co-authored-by: Andre Sailer <andre.philippe.sailer@cern.ch>

DDSim: compactFile can only be one instance, otherwise we cannot overwrite what is in the steeringfile, still takes multiple XML files

From https://stackoverflow.com/a/41153081

Needed for python2.7 compatibility

The XML files need to follow the structure where at the end is a closer file.
compactFile takes multiple arguments, or multiple instances of compactFile or both.

  ddsim --compactFile $DD4hep/examples/CLICSiD/compact/SiD_multiple_inputs.xml $DD4hep/examples/CLICSiD/compact/SiD_detectors_1.xml --compactFile $DD4hep/examples/CLICSiD/compact/SiD_detectors_2.xml $DD4hep/examples/CLICSiD/compact/SiD_close.xml --runType shell

Used when building doxygen for the website

    When using CMake 3.19 or newer and no CMAKE_CXX_STANDARD has been set before the call to find_package(dd4hep), downstream code will error with:
    ```
    CMake Error at /afs/cern.ch/user/v/vavolkl/cmaketest/tmp/CMakeFiles/CMakeTmp/CMakeLists.txt:13 (add_executable):
      CXX_STANDARD is set to invalid value 'CXX_EXTENSIONS'

    CMake Error at CMakeLists.txt:3 (try_compile):
      Failed to generate test project build system.
    ```
    because ${CMAKE_CXX_STANDARD} is empty. I couldn't find any relevant information in the changelog, so this might be a bug in CMake proper. When configuring dd4hep, CMAKE_CXX_STANDARD is assumed to always be set.

Also, don't add particle as daughter to its parents.

This addresses an issue with fully duplicated entries in the parents and
daughters relations for MCParticles. After this bugfix, only a single
entry for each parents is entered in the parents relations, similar for
daughters. More demonstrated behavior in pull request comments.

BEGINRELEASENOTES
- edm4hep: don't add particle as parent to its daughters, and vice versa
ENDRELEASENOTES

As it stands, this project requires to find a python version matching
the one ROOT was build against EXACTLY. This has been discussed in #842.
This adds an option `DD4HEP_RELAX_PYVER` that will accept **any** python
version. It will then perform a custom check if major and minor version
of the ROOT python version and the one found match (in which it should
be safe to assume ABI compatibility). If not, it prints a warning but
continues with the build.