Tests if start or and end of junc is on exon.
is_start_on_exon(tx_lst, jx_lst, intron_retention)
transcripts a GRangesList
with
transcripts defined as GRanges of exons.
splice junctions as GRanges objects in list format
logical vector indicating if junctions belong to an intron retention event
a logical vector indicating if start or and end of junc is on exon.