Add documentation (e.g. docstrings)

I realized that currently there is no documentation of any interpolation options. There is only the README on the github page, which refers to the PDF containing some of the derivations, but this seems to be incomplete in terms of documenting options, and pointing out some specifics of the options if applicable. So we could expand on some options that require additional information (like `Periodic()` BCs) in the README, or start writing docstrings. Any opinions?

This came up in #272, and it may after all affect only that `Periodic()` case, I don't have an overview if it's worth it writing docstrings for other options.