Catch the NaN error that occurs when histograms are not well-populated

[mrshirts]

When distributions are chosen poorly, it's very likely the ratio of the histograms will be undefined, as the denominator will be zero. This needs to be caught with a proper warning.

[mrshirts]

The issues show up as warnings:

checkensemble/checkensemble/checkensemble.py:910: RuntimeWarning: divide by zero encountered in log
ratio = numpy.log(hlist[1]/hlist[0]) # this should have the proper exponential distribution
checkensemble/checkensemble/checkensemble.py:911: RuntimeWarning: invalid value encountered in divide
dratio = numpy.sqrt((dhlist[0]/hlist[0])**2 + (dhlist[1]/hlist[1])**2)
checkensemble/checkensemble/checkensemble.py:910: RuntimeWarning: divide by zero encountered in divide
ratio = numpy.log(hlist[1]/hlist[0]) # this should have the proper exponential distribution

[ptmerz]

Migrated: shirtsgroup/physical_validation#1