Handle zero divs in consistent way

[douwekiela]

Sklearn has a consistent way of dealing with division-by-zero (either ignore, error out or warn), which we pass on in wrappers (see example).

In our own canonical evaluation modules, we also have potential ZeroDivs that we currently do not handle, e.g.: https://github.com/huggingface/evaluate/blob/main/comparisons/mcnemar/mcnemar.py#L94. We should fix this.