Add methods to Model for visualization

[nreinicke]

We currently have methods to visualize individual features and a combination of features. It would be convenient to add a method on the Model class that calls these functions (maybe using the feature ranges as default). Maybe it would look something like:

import nrel.routee.powertrain as pt

model = pt.load_pretrained_model("ICE")

model.visualize_feature("speed")
model.visualize_feature("grade")
model.contour("speed", "grade")

Where the user just inputs one or more feature names and the code does the work to generate the inputs for producing the plots.