FunctorCollection class: better _repr__ and __str__
This is more pythonic.
Example output for variables_jpsi = FunctorCollection({'LOKI_P': 'P', 'THOR_PT': F.PT})
:
# repr
<FunctorCollection: n_thor_fctors=1, n_loki_fctors=1>
# str / print
<FunctorCollection object at 0x7f67cad641c0:
LOKI_P: P
THOR_PT: ('::Functors::Track::TransverseMomentum{}', ['Functors/TrackLike.h'], 'PT')
>
Happy to discuss any tweaks :-).