Prepare BedTool for operations that require chromosome coords.

Set the chromsizes for this genome. If chromsizes is a string, it will be considered a genome assembly name. If that assembly name is not available in pybedtools.genome_registry, then it will be searched for on the UCSC Genome Browser.

Example usage:

>>> hg19 = pybedtools.chromsizes('hg19')
>>> a = pybedtools.example_bedtool('a.bed')
>>> a = a.set_chromsizes(hg19)
>>> print(a.chromsizes['chr1'])
(0, 249250621)