Adding lateral flow test intervention code.

[mattea]

Includes base code to test on symptoms and tracing, as well as a number of related parameters.

Note that the specific parameters (g/b) are currently untuned and not ready for production use.

This could have a few different alternate implementations, open to tweaks as desired, especially with regard to how the infectiousness curve is created and housed. Fine deleting the extra variables from the one_time_step_results too, but they were useful to have.

I have also added a bit of extra cleanup and parameters. If you need this separated out, I can.

• new_trace_token -> create_trace_token (for stricter build & c++ compatibility)
• misc whitespace cleanup
• quarantine_compliance_positive parameter - defaulting to 1 for backwards compatibility, although surveys show this to be lower in practive.

[mattea]

For some reason I can't respond to some comments, so adding it here:

• I added additional tests for specifically computing the number of tests performed.
• Switched the hardcoded peak to the calculated one.
• I couldn't switch to the new Model system, it broke file writing (e.g. model.write_individual_file())