API documentation incomplete & bugs

[kyleaoman]

The API documentation section has some incomplete docstrings particularly in the paicos.derived_variables section. There are also a number of errors/glitches in the html rendering on readthedocs, some examples are listed in #52 but please check carefully for more.

[tberlok]

Thanks for pointing out that the autodoc functionality had a lot of issues. I have fixed these as well as the ones that you carefully listed in #52.