Implemented const correctness for epsPhoton and epsParentPhoton

Summary:
Implemented const correctness for photon polarisation vector.

Implemented const correctness for epsPhoton and epsParentPhoton.

Implemented const correctness in rotateToHelicityBasis functions that use epsPhoton.

Updated History.md

Related with #13, !13 and T232.

Test Plan: Run all relevant tests and check that the output remains constant.

Reviewers: tlatham, jback, kreps

Reviewed By: tlatham, jback

Tags: #evtgen

Maniphest Tasks: T232

Differential Revision: https://phab.hepforge.org/D107