Eady equations and problems

[tommbendall]

The aim of this is to finally get the Eady equations/problems working again.

The main things to highlight are:

• the addition of two new equations
• two examples have been added to the examples directory
• there are several new diagnostics (many of which are useful more generally). These have been added to the existing diagnostics.py module rather than being in their own module
• the routines for setting the initial v field are now in the examples files
• the TrueResidualV diagnostic should still work, as we can point to the value of the velocity from the start of the timestep

[colinjcotter]

It's probably a bit dangerous to have N and Nsq both in the parameters with inconsistent values.