Add latex export for summary tables

[smmaurer]

There's been a feature request to add latex export for summary tables.

OLSRegressionStep

This uses the core UrbanSim regression class, urbansim.models.RegressionModel(). We save the model object for interactive use, which in turn exposes the StatsModels results object (model_fit, bad name), which provides latex export.

So you can get a latex version of the summary table like this:

m.model.model_fit.summary().as_latex()

BinaryLogitStep

This uses StatsModels Logit. We don't currently save the results object, but saving it for interactive use will be an easy change. That will expose the StatsModels latex export.

SmallMultinomialLogitStep

This uses PyLogit MNL via ChoiceModels. We save the PyLogit model object, but it doesn't look like this provides latex export of the summary table. Need to look into this further.

LargeMultinomialLogitStep

This uses the ChoiceModels estimator, which has custom-built summary tables. We can look into replicating the StatsModels approach for latex export.

Perhaps we should make another try at using the StatsModels summary table generator: http://www.statsmodels.org/devel/generated/statsmodels.iolib.summary.Summary