Increase unit test coverage

[cmacmackin]

Currently unit test coverage is a little over 20%. We should aim for this to be much higher: ideally over 90%. Below are a number of methods/functions which are currently untested but for which it is relatively easy to write tests:

• All of the div_ops functions
• The outputVar methods on components (check that the expected variables are present)
• classical_diffusion
• evolve_energy (can model tests off of those for evolve_pressure)
• ion_viscosity with perpendicular flow included

The following are large components with little or no testing. I'm not sure how easy they'd be to test, but it would significantly increase coverage levels.

• detatchment_controller
• neutral_boundary
• neutral_full_velocity
• neutral_mixed
• neutral_parallel_diffusion
• recycling
• sheath_insulating_boundary
• vorticity

[dschwoerer]

I think the div_ops should be MMS tested.
Same for the physics components, I think it would be much better to write MMS tests, rather then unit tests.

[cmacmackin]

We should write both. Unit tests are useful because they are so quick to run.

[ZedThree]

I've ported some of div_ops to BOUT++ with testing: boutproject/BOUT-dev#3200