Fixed probmax issues and bugs in EvtXPsiGamma model.

Summary:
Defined separate calcAmp function, implemented scan for initialisation and weighting to compensate for phase-space changes due to resonance width.

Implemented const correctness and fixed bug in couplings of rho and omega.

Corrected loops over photon and vector states.

Improved error messages. Removed useless info messages. Renamed variables for understandability.

Changed static const masses to get mean mass as rho and omega have large widths.

Resolved minor conflict in test file for X38722-+_PSI_GAMMA model.

Updated History.md.

Test Plan: Run tests for EvtXPsiGamma model. Distributions should not change much from master branch though.

Reviewers: tlatham, kreps, jback

Reviewed By: tlatham

Differential Revision: https://phab.hepforge.org/D102