Transcript::add_Exon now throws if exon could not be added b/c of overlap with other exon, rather than silently doing nothing

*do not create translation if less than 3 nucl. left in CDS

bug fix: still adjust coords for exon that is wholly consumed by introduced frameshift intron b/c coords are used later for transcript splitting.

added a sub_Slice method, that returns a smaller slice. This will not make use of the cached features of the parent slice though

just gene, transcript, and exon table;