test if junction is canonical junction
is_canonical(junc_id, ref_junc, exons_gr)vector of junction id to test
vector of canonical reference junctions
GRanges of canonical exons
logical vector of same length as junc_id indicating
if the input junc_id is in the set of reference junctions ref_junc or
consists of two directly adjacent positions (intron retention) and
overlaps (completely within) a canonical exon from exons_gr.