save 'None' ytransform as empty string instead of null
fix corresponding unit test
fix bug in config saving
undoing commits before the last one
Merge pull request #87 from synthicity/ytransf-mapping-null

save 'None' ytransform as empty string instead of null
Fix docstring indentation in regression.py.

Also use print function so my linter stops complaining at me.
Save final simulation data under "final/" namespace in HDFStore.
Initial docs on the UrbanSim simulation framework.

[ci skip]
Document class attributes in urbansim.sim.simulation

[ci skip]
Make _TableSourceWrapper a private class.