[WIP] Parse Separations and Mixer archetypes properly

[nuclearkatie]

Parses Cycamore Separations and Mixer facilities separately than other archetypes due to different xml input schema. Previously, they would be parsed but with miss either input or output commodities (input for Mixer, output for Separations).

Closes #22

[gonuke]

It would be good to add a test by changing your standard XML file to include a separations and mixer archetype.

We can also discuss some design elements here - might be really good for a S/W meeting!
Thinking ahead to other archetypes (e.g. not in cycamore) that might have unusual signatures, I wonder if there is a way to generalize this somewhat?

[nuclearkatie]

Agreed, I think there are several ways to do this and I chose the most straightforward simply because I wanted to immediately use the added ability to parse separations/mixer in an example problem. I can bring it to software this week