Performance Update

The original source code for TFM-Pvalue had some issues with dynamic memory allocation that were leading to pretty severe memory leaks in pytfmpval when used iteratively for many motifs. I went back and cleaned up several instances where pointers weren't properly dereferenced, so this issue should be mostly fixed (or at least so minimal that it won't be noticeable).

A fail-safe for high-memory usage was also built into the tfmp module, but it shouldn't ever be hit unless you're using some ridiculously long, degenerate motifs. If it occurs, an approximate p-value/score will be returned instead.