Determine if we should process this chromosome: needs variant regions defined.

def has_variant_regions(items, base_file, chrom=None):
    """Determine if we should process this chromosome: needs variant regions defined.
    """
    if chrom:
        all_vrs = _get_variant_regions(items)
        if len(all_vrs) > 0:
            test = shared.subset_variant_regions(tz.first(all_vrs), chrom, base_file, items)
            if test == chrom:
                return False
    return True