Cover more combinations of functionality in the tests

[DillonJ]

Is your feature request related to a problem? Please describe.
Tests cover only a limited combination of SpineOpt functionalities

Describe the solution you'd like
Cover more combinations of functionalities

Describe alternatives you've considered
N/A

In some recent commits to SpineOpt, there were issues with fix_ratio constraints because the tests did not test a sufficient range of possible combinations of fix_ratio parameters. A possible solution is to increase the complexity of the example systems. Specifically, there should be units which have multiple inputs and outputs and have fix_ratio parameters defined on multiple combinations of flows.

Generally speaking, we could increase the complexity of the example systems.