Fix auto-formatting NaN

[darkdragon-001]

Current version fails with error

ValueError: cannot convert float NaN to integer

This is because round(float('nan')) does not work. This pull request adds a special check to auto formatter and uses text format for NaN instead (string 'NaN').
Maybe it would be better to display an empty field though (fn = lambda x,**kw: '')?

---

I would like to open the discussion to implement an additional argument for a NaN-formatter which could be a function to convert Nan to string.

[foutaise]

Hi @darkdragon-001

Thanks for this PR. I prefer to display an explicit 'NaN' instead of an empty string.
A new release will follow soon.

Best regards.