Add checks for metrics

[LacombeLouis]

Would be good to add additional checks for the metrics.
For example:

• check that y_true and y_pred are the same length
• check that there are no Nans

Simple check to assure that the user has good feedback upon error in metric.

[JumpingDino]

Is it okay if I try to do this one?

I expect to create two functions

• check_array_length
• check_array_nan

which will try to make these checks for the metrics inside metrics.py

[thibaultcordier]

Hello @JumpingDino, that's a good suggestion for solving this issue, thank you.
You can fork the repositoty (see CONTRIBUTING.rst) and propose a pull request with these functions in the mapie/utils.py file and their corresponding tests. Don't hesitate to ask for help during implementation.