Updated environment and Pandana 0.4 compatibility

[smmaurer]

This PR has several small fixes:

1. Adds cast = True in update_col_from_series(), in the household_relocation step. I was getting an int32/int64 error there. (Already fixed in another PR.)

2. Replaces settings injectable with newer policy injectable in one spot in summaries.py -- I was getting an error that settings doesn't exist, because it's no longer passed into that model step. (Everything else in that model step uses policy; maybe this line was broken when I was resolving conflicts during the merges I made a couple of months ago?) But @theocharides please confirm that this looks right! (Already fixed in another PR.)

3. Adjusts some Pandana aggregation settings -- it turns out that 'average' was an undocumented alias for 'ave', and 'average' was accidentally removed in Pandana v0.4. Using 'ave' will work with all Pandana versions. See Reinstate alternate names for aggregation types? UDST/pandana#133.

4. Environments: Updates requirements-2019.txt to improve support for Python 2.7.

5. Adds a Conda environment file: baus-env-2020.yml. This should be the default now.

6. Updates main README.md with new installation instructions.

[theocharides]

Hey @smmaurer- looks like you already caught that # 1 was fixed in master, as was # 2, prior to this PR.

[smmaurer]

Hi @theocharides, this is ready to merge! I've added installation instructions, and updated the title and original writeup.

My only uncertainty is whether we should still include python baus.py --mode preprocessing in the installation instructions -- see Slack. I've left it out for now, but we can add it back if needed.

[smmaurer]

Ok, I've added back the preprocessing to the instructions! There's a bug preventing that from running at the moment, which I'll fix in another PR. But no need to wait on that for merging this one, @theocharides.

[theocharides]

Confirmed that the Conda environment works in Linux (in addition to @smmaurer's Mac testing). Will likely remove the need for a pip install option going forward.