balance_cooler allows store_name even when store=False

[gfudenberg]

Should this raise an error? https://github.com/mirnylab/cooler/blob/c9c718fdebccbda41ad10c47f700853f79ee3cd3/cooler/balance.py#L271

[nvictus]

I'm okay with this. It provides an explicit default. The alternative would be to have a hybrid store argument that takes a bool or a string. store=True would use an implicit default name of "weight" and store={name} would use the provided name.