Fix precipitation

[tommbendall]

This attempts to fix precipitation (issue #334) by:

• ensuring that the advection term associated with Fallout has its transport label removed
• adding the prognostic label to that term

It also attempts to resolve some outstanding physics problems by:

• allowing the Timestepper to take physics schemes
• the PrescribedTransport time stepper is now only different in allowing the wind to be time-varying

I also implemented a routine to return an equation's active_tracer object for a particular species, which should help with error checking in the physics.

[atb1995]

The changes look sensible to me. My understanding/summary of the changes is:

• get_active_tracer allows you to get meta data of a field. In the instance of Fallout this is to check if the tracer is a mixing ratio
• The bug in Fallout is fixed by removing the transport label and adding the labels prognostic and physics. prognostic mean's it is counted in time stepping for physics, whilst removing transport prevents double counting.
• In the case of PrescribedTransport, a physics time stepper is no longer required. I'm assuming this because having a separate time stepper does not make sense in this case?
• Timestepper now has physics schemes as well, but with the error thrown if a more complex physics time stepper is used.
  Happy to approve this pull request.

[tommbendall]

Thanks Alex! Yes that's right about the physics not wanting a separate time discretisation for the PrescribedTransport