Added final flank expansion functionality back in

Corresponds to PR #93; version 0.5.14.0 accidentally removed the final motif flank expansion that was controlled by the parameter "final_flank_to_add", such that the flank expansion was effectively 0 (note: this only affected the flank expansion that was done at the very end of the tfmodisco pipeline; there is still flank expansion controlled by the parameter "initial_flank_to_add"). I added the functionality for final flank expansion back in, and for backwards compatibility with version 0.5.14.0 I have set the default value of final_flank_to_add to be 0 (previously, it was 10; a default of 0 is actually probably better from a user perspective, because sometimes users run tf-modisco on very short sequences, and having a large final_flank_to_add can cause many seqlets to get discarded when the expansion extends beyond the end of the sequence). I also cleaned up some of the notebooks.