Write the DF as rds file

[soheila-sahami]

Thanks again for the applied change to read the rds files.
Is it possible to add a new feature to write the df as an rds file too? In this way, there will be a full bridge between R data and Python.

[vnmabus]

I think that writing is more difficult, as the conversion from Python objects to R s-expressions is necessarily lossy. That said, it could be possible to allow the user to use their own conversion routines for custom classes or whatever.

For now I am too busy to implement this, but if anyone is interested in trying a PR, I will review it.