split out generate_dataset operations

[robfitzgerald]

i generated a dataset which completed most steps but failed on the last due to an SSL error (proxy settings):

the code to run just the model import is not a separate function, so i can't simply re-run that step. nor can i specify which steps i want to run as an argument for the generate_compass_dataset function. a refactor to separate phases out and make it possible to automatically find the next step to run or name it explicitly at generate_compass_dataset would make this easier:

generate_compass_dataset(g, "output_dir", phases=None) # default behavior

generate_compass_dataset(g, "output_dir", phases=["grade", "powertrain"])  # re-run a few phases

generate_compass_dataset(g, "output_dir", phases=["powertrain"])  # re-run just that last one