BUG: Fix bug in make_distance_comparison_plots

This bug would create empty groupings in a plot when sample ids appeared in
the mapping file but didn't appear in the distance matrix.

The code also checks for cases where samples are not shared between mapping
file and distance matrices.

Fixes #1534 
Looking more into it, the code supports a single sample and has tests for it, so I'm gonna revert to the way it was i. e. checking for at least a shared sample.