Plotting and analysing metrics?

[ddundo]

I thought it would be nice to add some plotting functionality to animate for metrics. I mentioned this to @acse-ej321 on the last meeting since she showed nice figures in the past.

And potentially related to this: I feel like the density_and_quotients method of RiemannianMetric is a bit out of place there. All other methods are used to actually construct the metric, while density_and_quotients is never called, and is only used for analysis, right?

So I feel like it would be nice to have something similar to quality.py for meshes, but for metrics. And the ability to plot them easily.

[jwallwork23]

So I have been thinking about this a bit - it links to #37. Since the RiemannianMetric class is now quite bloated, it would be nice to introduce a superclass called SPDFunction, which is a subclass of Function for the symmetric-positive definite case. Then it would make sense to move (e.g.) enforce_spd, assemble_eigendecomposition, compute_eigendecomposition, density_and_quotients there. We could also make the P0Metric class used internally a subclass of SPDFunction but not RiemannianMetric.