Speed up transition models with linked tables

Biggest speed improvement was specifying explicit copying of
new linked table rows so that pandas didn't do any
"are you assigning to a copy?" checks. More speedups were achieved
by using a groupby on the linked table rows that needed copying to
cut down on filtering done. Overall speedup is more than 10x.
Change return values of mnl_estimate

Now returns a dictionary of log likelihood values
(keys: 'null', 'convergence', 'ratio') and a DataFrame with
columns 'Coefficient', 'Std. Error', and 'T-Score'. The
DataFrame is not given any special index in mnl_estimate,
but when it is used with MNLLocationChoiceModel it gets an
index from the design matrix column names return by patsy,
which are derived from the model patsy expression.
Add function for converting Series to yaml-able

Converts index and values separately to native types and
zips them together into a dict.