Allow user to provide their own buildstock.csv file

[nmerket]

Fixes #30

@aspeake1 I haven't done any testing on Eagle for this one (system time). Can you test it out and see if it works?

The way to use it is to add a buildstock_csv key to the baseline group.

baseline:
    buildstock_csv: /path/to/your/buildstock.csv
    # comment this line out because it can't use both and will error out if you try.
    # n_datapoints: 350000 
    n_buildings_represented: 80000000

Can you also update the documentation to reflect this change? It's in the docs/ folder. See the restructured text primer on the markup format. PyCharm is generally good at autogenerating the preview of it. Otherwise the rendered docs will show up in the artifacts of the CircleCI build. Let me know if you have questions.

[nmerket]

I tested this on both Eagle and locally using docker. Seems to work as expected.

@aspeake1 Go ahead and use this branch for your analysis and when you're satisfied that's it is working, approve it and I'll get it merged. No hurry.