Ratio in map comparison fails if reference hist has zero nominal values and is not a bare numpy array

[thehrh]

Setting np.errstate in https://github.com/jllanfranchi/pisa/blob/dev/pisa/core/map.py#L521 doesn't help when hist is an uncertainties.unumpy.uarray (I think this has been an open issue for a long time: lmfit/uncertainties#12). This can be fixed by applying set_errors(None) to the reference map before the division is performed.

[thehrh]

This is actually not sufficient, since np.errstate only effective when both maps that are compared have no uncertainties defined (in the case of a zero in the reference map).