Clean parse_lombardi; Rename model config dir

[brynpickering]

Fixes #17
Fixes #16
Fixes #15

A number of changes here:

1. I've made the preprocessing an automated part of loading the italian model. It makes it possible to set custom investment step resolutions / periods and have that information filter through.
2. I've removed some CSV files in favour of defining the data in YAML (where it is the same value in every investment period).
3. I've brought in generating some simple CSVs into the pre-processing step so that changes in investment resolution is passed to them.
4. I've changed from models to model_configs to not get name clashes with the models.py file.
5. I've cleaned up the import and export of electricity from outside italy so that it is actually used by the model. This entails adding custom math to link two technologies.

This does leave parse_ppm.py by the wayside, but that's fine for now I think. It's still possible to run it from the command line (as it is with parse_lomabrdi.py).

[irm-codebase]

I agree on leaving ppm on the side. I was trying to get it to work so that we could change the start year (i.e., 2010, 2015, 2020), but its accuracy is... disappointing. For later, maybe.

[irm-codebase]

Apologies for the bugs Bryn. Things look great, just a couple of nice-to-haves.

Approved