Thermal sw linear solver

[nhartney]

This adds a linear solver for the thermal shallow water equations to linear_solvers.py. It allows the use of the semi-implicit quasi-Newton time-stepping scheme with the thermal shallow water equations (and implementations of the moist shallow water equations that include thermal effects).

The solver follows the same approach as the IncompressibleSolver. The strategy is as follows:

1. Eliminate buoyancy.
2. Solve the resulting system for velocity and depth using a hybrid-mixed method.
3. Reconstruct buoyancy.

The thermal_williamson2 example now uses the semi-implicit quasi-Newton scheme.

[tommbendall]

This looks really good to me. I have no suggestions so I think we could merge this straightaway!