BUG: paired-heatmap keep_top_samples option is broken

[nbokulich]

if keep_top_samples=False, get an error that "select_microbes is referenced before assignment". Looks like the alternative broke at some point and this line just needs an else statement:

https://github.com/biocore/mmvec/blob/a520b32d8cf56a1248a142cb332558ca462b1f71/mmvec/heatmap.py#L187-L189

reported in #120