feat: calculate change table MW-miles with tests and mocks

[danielolsen]

This branch adds a method to the Analyze object to calculate the aggregate MW-miles of upgrades from a given change_table. A new folder tests/ is created with a new file mocks.py for common testing (copied from PostREISE) and a test script test_calculate_mw_miles.py to test returns from the new function for data structure and proper values.

[rouille]

Is there any reason why it is not in postreise/analyze?

[danielolsen]

@rouille to calculate the difference between the scaled capacity and the new capacity, we need to be able to access the original capacity, and the only place I saw that ability was a hidden attribute in the grid object (._original_grid). So I figured if we're accessing this, I figured we should access it from the same package as we define the Grid object. I'm agnostic to where this code lives though, if you think it's better to have it in postreise I'll put it there instead.

[rouille]

Yes I have seen. It does not feel right to include the analysis there. I would rather pass a scenario instance to your function and:

• use get_ct() to get the change table
• get original grid as follows: original_grid = Grid(s.info['interconnect'].split('_')). You will need to import the Grid object
  How does it sound?

[danielolsen]

I am okay with your approach. I'll close this PR and make another one with the revised function in postreise/analysis.

EDIT: See new PR here: Breakthrough-Energy/PostREISE#41