Updated docs for BaseIterator._params_lookup

[pfebrer]

Wrote the docs on how to use BaseIterator._params_lookup.

By the way, have you thought about cleaning up the docstrings by using a first line that gives a hint of what the function class does, then a blank line, and then the rest of docs? I think it is much easier to the eye, and I believe is also the standard for python :)

[pfebrer]

However I would reduce the white spaces in favor of indentation when you describe the various keys of _params_lookup.

I don't understand exactly what you mean, so you can change it if you want :)

Should we mention in the class docstring that _params_lookup is a tuple and the groups are checked in order?

Yes! This is important, I will change it.

If you run yapf -i on the two files changed, the pre-commit error will disappear.

Will do!

[bosonie]

I saw few more errors in the docstrings (the parts I wrote). Therefore I will review them again separately. If you just fix the pre-commits and add a line about _params_lookup being a tuple, I will approve and merge.

[pfebrer]

Done!