More efficient testing for gene overlap

[standage]

Currently, the AgnLocusStream tests for overlap by comparing a gene's location to those of all genes in the current buffer. Karin Dorman correctly pointed out that this is unnecessary and that simply testing whether the buffer's rightmost position exceeds the gene's leftmost position is sufficient.

I don't expect this will affect performance all that much, but it is a cleaner and more efficient solution and easier to describe as well. Will implement and test this soon.