Make matplotlib an optional dependency

[kinnala]

Starting work on #286 .

[kinnala]

@gdmcbain can you make a quick review at this point? Comments on the overall structure would be appreciated and if you have any ideas or alternative implementation strategies.

I'll continue with fixing rest of the examples after you agree this is the right way to go forwards.

[gdmcbain]

• Should skfem.visualize itself be a package like skfem.importers with skfem.visualize.matplotlib underneath it like skfem.importers.meshio? This might facilitate later adding other visualization backends like yt, pyvista, holoviews, …. Matplotlib is obviously primary for its ubiquity and good enough for most one- and two-dimensional work but it distorts three-dimensional views.

• Instead of the chains of if (isinstance, ...) would abc or functools.singledispatch be tidier?

[gdmcbain]

Another thought for the further future: Thus far the visualization is based on the skfem.Mesh class hierarchy but it also partially implies ElementLineP1 for MeshLine, ElementTriP1 for MeshTri, etc., or sometimes P0, Q0, etc., I think. Some visualization backends (Gmsh, ParaView?, ...) might support higher order elements. Does eventually supporting higher order visualization imply anything about the structure of the skfem.visualization package? Should the functions take bases rather than meshes? (Noting that an skfem.assembly.GlobalBasis has a .mesh attribute anyway.)

[kinnala]

Fantastic suggestions, as usual. Here is a notebook I used during development.
plotting.pdf

I'm going to implement also the possibility of drawing through InteriorBasis since I removed the plotting of piecewise-constant and DG functions and refinterp should be used instead. I was thinking syntax like this:

plot(interior_basis, solution)

and the call is the passed to interior_basis.refinterp and, e.g., plot(m: MeshTri, ...).

[kinnala]

Next I start fixing the examples. It might take a while.

[kinnala]

Starting by running the Makefile under docs/examples and fixing all errors. Later fixing the docs.

[kinnala]

Starting to be ready for review. Fixes #286 .

[kinnala]

Am I missing some examples?