Process dataframe docstrings

[ericmjl]

@a-r-j requesting a short review on this one.

I added docstrings to the process_dataframe function, and slightly modified its implementation for simplicity.

One thing I'm a bit unsure of: if granularity == something shows up in two places. Do you think this can be simplified? "granularity" as a category of properties to worry about -- if it can be handled in one block of code rather than two, that would help with reasoning about the function and debugging/maintenance later on.

[a-r-j]

LGTM, thanks Eric!

I think you're right about granularity - it's doing too much. The idea is to have control over:

1. What the nodes are in a residue graph (e.g. a-Carbon "CA", b-Carbon "CB")
2. Whether to use that atom position or compute residue centroids
3. Whether or not to build a residue-graph as above or an atom-graph

[ericmjl]

@a-r-j ok! That's definitely for another PR. Gonna raise an issue so that we can track this. 😄