Summary:
Rename member variables to follow convention of `m_` prefix

In addition:
* Introduce pre-build check in the CI that the member variable naming convention is being followed in all classes
* Fix some shadowing of variables
* Fixes to Doxygen configuration
* Enable shadowing and overloaded-virtual compiler warnings

Test Plan: Run tests for all models using the results of the same tests on `master` as reference

Reviewers: jback, averbyts, kreps, abudinen

Reviewed By: jback, abudinen

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D58