23.0-reduce_CC2-PixelDigitization

Following encouragement from @jchapman that the previous complexity-reducing MR had produced a small performance gain, this MR continues to its logical conclusion and reduces the cyclomatic complexity of the relativeBunchXXXX members of FEI3SimTool to 12, and hopefully should also improve performance. Some other minor tweaks include moving some variable calculation outside of loops where possible. The overall code length is also consequently reduced, from 1009 lines to 675 lines (for this class).