marco-mariotti:
- bug fix on subsequence and spliced_subsequences, occurring when different groups of intervals overlapped, making the not contiguous in the sorted df
- expanded sort: now accepts "5" for sorting ascending on + stand, descending on - strand
- expanded gtf/gff readers
- expanded gtf/gff writer, now accepts a map_cols argument
- new method: boundaries to get boundaries of group of intervals
- expanded extend, now optionally accepts a group_by argument
- changed name of get_fasta to get_sequence
- new method: get_transcript_sequence
- docstring modified in a few places