Implement `const` correctness for `epsPhoton`, `epsParentPhoton` and related functions for photons. See [#13](https://gitlab.cern.ch/evtgen/evtgen/-/issues/13). Solved in D107.
Protect polarisation vectors for vector and tensor particles against wrong indices. See [#12](https://gitlab.cern.ch/evtgen/evtgen/-/issues/12)