BUG: group_timepoints fails when using Beta Diversity with a single donor

[lizgehret]

The following code within _engraftment.py needs to be modified to allow for single donor within the metadata and diversity measure:

# HELPER FUNCTION FOR GroupDists[Unordered, Independent]
def _independent_dists(diversity_measure, metadata,
                       used_references, is_beta, used_controls):
    unique_references = sorted(used_references.unique())

    if is_beta:
        try:
            ref_idx = pd.MultiIndex.from_tuples(
                itertools.combinations(unique_references, 2))
        except TypeError:
            raise TypeError('Single reference value detected. More than one'
                            ' unique reference must be provided for'
                            ' successful grouping.')

[ebolyen]

The approach we'll take is not producing rows for the reference group in the reference distribution. This may result in an empty table if there are also no controls.