1D shallow water equations

[jshipton]

This PR adds an equation class for the 1D shallow water equations and the required 1D transport and diffusion terms.

To Do:

• make example into a KGO test instead of an example
• define linearisation
• this won't work with SIQN without another bespoke linear solver - I'd rather not write that!

@tommbendall any comments / requests?

[jshipton]

aha, we used to make the velocity a vector with just one component... @tommbendall would you like to keep it like this (to test the usual code for the transport forms) or shall I change this test to use the new 1D forms?

[tommbendall]

It's great that we'll finally be able to do this! As well as the picky individual comments, some broad thoughts:

• as we discussed offline it would probably be better for the normal form to return the 1D form if (a) the mesh is 1D and (b) the velocity is a scalar. Hopefully this will ensure that the tests pass
• could you rename the example to shallow_water_1d.py or something similar, for some reason I don't like it starting with a number!

[tommbendall]

I'm happy with this now! I'll leave it un-merged so that you can do it if you think that it's ready