Retrieve gender for a batch of items if consistent.

    Better not to specify for mixed populations, CNVkit will work
    it out
    https://github.com/bcbio/bcbio-nextgen/commit/1a0e217c8a4d3cee10fa890fb3cfd4db5034281d#r26279752

def _get_batch_gender(items):
    """Retrieve gender for a batch of items if consistent.

    Better not to specify for mixed populations, CNVkit will work
    it out
    https://github.com/bcbio/bcbio-nextgen/commit/1a0e217c8a4d3cee10fa890fb3cfd4db5034281d#r26279752
    """
    genders = set([population.get_gender(x) for x in items])
    if len(genders) == 1:
        gender = genders.pop()
        if gender != "unknown":
            return gender