Managing the urbansim dependency

[smmaurer]

The templates library has urbansim as a dependency right now, for utilities and some OLS regression code, and it's causing problems for conda installation.

This is because we're exploring moving UDST libraries to the conda-forge channel, instead of maintaining our own udst conda channel. (That should be easier for users, and will probably make the release process easier too.) Currently, orca and choicemodels are on conda-forge, but urbansim is only on udst.

Conda Forge is pretty strict and doesn't seem to allow recipes with dependencies from other channels. So to include a proper recipe for urbansim_templates, we need to put urbansim on Conda Forge as well. To get this working, it looks like we'll need to update urbansim to remove its zbox dependency (which is not on Conda Forge either), as was done for Orca in UDST/orca#32.

Alternatively, we could eliminate urbansim as a dependency.

Short-term strategy

In the meantime, i've gotten urbansim_templates to build on Conda Forge simply by not listing urbansim as a requirement. Users will need to install it separately.