SeqID union, ParsEval AgnLocusParser utilization

This revision implements two improvements: first, I implemented the `agn_seq_union' function to complement (no pun intended) the `agn_seq_intersection' function, and have replaced the use of the latter in the `AgnLocusIndex' class with the former; second, I have updated ParsEval to utilize the `AgnLocusParser' class. Not only does this drastically reduce the complexity of the ParsEval program, but it also makes the reports much nicer in that they report all genes, mRNAs, etc, rather than just those residing on shared sequences.