rollout_seq_features will roll out the SeqFeatures from a SeqRecord.
EncodingAlphabet now can optionally take a gap_characters set of characters to add to the
alphabet letters. It also takes add_lower_case_for_inserts which will duplicate the alphabet,
but convert the letters to lowercase.
Fixed bug in GenomicDataset.from_path where it still referred to init_from_path_generator.
EncodedSeq now supports iterating through kmers, see EncodedSeq.rollout_kmers for options.