Resolves issue #686.

Updated 3 lines in compare_alpha_diversity.py to check if p_val or obs_t return as None because of low variance data passed to mc_t_two_sample. Updated the script documentation to identify this third source of error (why you would get None, None for pval, tval).
Merge pull request #689 from wdwvt1/issue686

Resolves issue #686.