Revamp unit tests after #78, #80, #83 are complete

[Alomir]

#78 #80 #83

After the above are complete (especially #78, the others are not strictly necessary but will cause enough code churn that they should be done first), we will be in a position to revamp the unit tests:

1. We will no longer need modelStructures.h in the test directories, as it should be gone from src/sipnet. We should remove those files, and any infrastructure geared around them (check the Makefiles)
2. As suggested in this comment, we should create - and then use - a set of reference inputs for testing. The goal is that those inputs would give reasonable coverage of base sipnet functionality without being too extensive. Hopefully, most positive tests would use those inputs, though negative tests will almost certainly still need their own.
3. As part of the above, create a markdown file in /tests/sipnet (most likely) that documents these reference inputs.

[dlebauer]

As part of the above, create a markdown file in /tests/sipnet (most likely) that documents these reference inputs.

I was thinking these could be referenced from the file format descriptions in docs currently in parameters.md (but will eventually be a separate file #99), perhaps this would allow us to keep the docs in one place? And / or were you thinking something more specific than user docs would go in /tests/sipnet?

[Alomir]

Not sure what I was thinking, the new markdown file should definitely go in with the other doc files.

[Alomir]

#114 will achieve part of this ticket, but not the reference inputs.

[Alomir]

As far as reference inputs go, I'm treating this mostly as 'reference inputs for smoke tests', as unit tests by and large either don't use inputs or require customized ones.