Add dictionary of variable names to output of linearize() function when using Python

[casella]

The current implementation of linearize() when using Python output only provides the ABCD matrices.

@naibafomsare proposed here to add a dictionary with names for inputs, outputs, and states. This should be done to get the same functionality as in the Matlab output

[arun3688]

@casella the feature is implemented with this commit 62abb40

[casella]

@naibafomsare, would you mind testing it with the next nigthly build and report? Thanks!

[naibafomsare]

@casella and @arun3688, I have tested the new code, and it seems to be working very well, thank you very much! I can simplify my code now.

The only observation I have is about the description of the new feature. @arun3688 did not add a dictionary, but just a list with the names of the variables in the appropriate order. Such a solution is better than the one I had proposed.