BUG: paired-heatmap keep_top_samples option is broken

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

Start in mmvec/heatmap.py at lines 187-189, where the paired-heatmap keep_top_samples option handles sample selection. Reproduce the error with keep_top_samples=False and inspect the alternate path for select_microbes. Done means that option completes without the referenced-before-assignment error.

Written by the indexing model from the issue text.