This function takes a vector of condensed modification probabilities
(e.g. c("128,0,63,255", "3,78,1") and a vector of directions (which
must all be either "forward" or "reverse", not case-sensitive),
and returns a vector of condensed modification probabilities where those
that were originally forward are unchanged, and those that were originally
reverse are flipped to now be forward.
Called by merge_methylation_with_metadata() to create a forward dataset, alongside
reverse_sequence_if_needed(), reverse_quality_if_needed(), and reverse_locations_if_needed().