RDF accepts vacuum trajectories, empty selections, and delta_r = 0, producing NaN output or bare crashes

[galjos]

Four ways to get garbage or an unhelpful crash out of the RDF:

vacuum trajectory          -> run completes, g(r) = [nan nan inf inf ...]
reference selection "Na"
  matching nothing         -> run completes, g(r) all NaN
same on the legacy path    -> ZeroDivisionError: float division by zero
delta_r = 0.0              -> OverflowError (legacy) / ZeroDivisionError (general)

A typo in a species name yields an all-NaN result file with exit code 0.