Helper functions for best track selection

Yet another low-hanging fruit: best_track(...), best_jmuon(), best_aashower() etc. are missing.

See the reference implementation in the KM3NeT DataFormat or in km3io

added easy enhancement labels

To follow the Julia designs, I left out the underscore (which I hate anyways )

besttrack() is done in fc27e7a4

created branch 21-helper-functions-for-best-track-selection to address this issue

mentioned in merge request !7 (closed)

Done in b56c643d

closed