Retrieve list of locations to analyze from input BED file.

def _get_location_list(interval_bed):
    """Retrieve list of locations to analyze from input BED file.
    """
    import pybedtools
    regions = collections.OrderedDict()
    for region in pybedtools.BedTool(interval_bed):
        regions[str(region.chrom)] = None
    return regions.keys()