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](https://gitlab.cern.ch/evtgen/evtgen/-/issues/13), [!13](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/13) and T232.