Rename member variables to follow convention of `m_` prefix
In addition:
* Introduce pre-build checks in the CI to
- Check that clang-format has been applied to all files
- Check that the member variable naming convention is being followed in all classes
* Remove unused class EvtDecayParm
* Remove unused var from EvtXPsiGamma
* Fix some shadowing of variables
* Fixes to Doxygen configuration