Move parameters from code to input configuration file

[trevorhardy]

There are several scripts that have many hard-coded parameters scattered through the source code (feeder_generator.py, prep_substation.py). To improve transparency and maintainability, we should move these parameters out of the source code files and codify them in input configuration files. Having them as a separate file in the code base allows for better separation between code and input data as well as provides a more direct way of documenting those input parameters (units, where they came from, why they are needed for certain functions).

[trevorhardy]

Largely complete in DSO+T feeder generator; close enough for us.