A package of idealised physics routines

[tommbendall]

This PR adds a package of idealised physics routines, that are largely taken from the DCMIP (2016) test case documentation.

1. A general SourceSink physics scheme, which takes some expression and evaluates it as a source or sink of a variable.
2. A SurfaceFlux scheme, which allows prescribed surface fluxes to be applied to theta and water vapour.
3. A WindDrag scheme.
4. A StaticAdjustment scheme
5. A BoundaryLayerMixing scheme

Additionally it implements a Firedrake "escape hatch", allowing us to break out of the standard structure of Firedrake Functions and to handle column-wise data instead. This is the only way to implement some physics schemes (such as dry static adjustment) and opens the route to us doing other non-FEM algorithms in the future.

Lots of other features have been pulled off this branch and brought onto main through separate PRs, but this branch still includes:

• a bit of a hack to allow ExplicitMultiStage schemes to work when there is no right-hand side

[jshipton]

just a few minor comments - this opens up a lot of cool stuff!