Add drop_nans method to DataFrame and LazyFrame

[jeromepl]

While investigating #117 I found that the #drop_nans method was missing.
It's implementation is basically identical as #drop_nulls, which made this an easy addition.

[ankane]

Hi @jeromepl, thanks for another PR! Please make the docs and examples consistent with the Python library.

[jeromepl]

@ankane the docstring examples have been updated to match py-polars

[jeromepl]

There are a couple more examples in py-polars, but that don't test the drop_nans method. They are there only for documentation to show how to get similar behaviour using other Polars methods. I omitted them here, but let me know if they should be added

[ankane]

Thanks @jeromepl, looks great. There's a conflict after merging #117, but can merge once that's resolved.

[jeromepl]

@ankane rebase complete!

[ankane]

Awesome, thanks!