DFReader pandas module

[robertlong13]

WIP

I was tempted to make this as a totally separate repo, but maybe it's the right fit here. Opening a draft PR in case anyone else is interested in it.

This is handy for writing analysis scripts and jupyter notebooks. It resamples all your fields of interest to a common time base and returns everything together as a pandas dataframe. I also have it cache that dataframe so that you can quickly rerun your scripts during development without needing to handle caching manually.

[tridge]

@robertlong13 looks good, just needs to be out of draft to merge