Divide by zero error handling in map.py

[ts4051]

In map.py functions such as compare, there are cases where one map is divided by another. If the denominator map has any bins with 0 entries, this fails with a divide by zero error.

This is handled for the case of numpy arrays, but when the bins have uncertainties the error appears due to lack of robust support for divide by zero in the uncertainties module.

[thehrh]

cf #479