Add support for visualizing 2D meshes as svg

[kinnala]

This PR enables visualizing 2D meshes as SVG without matplotlib and its now used by default in a Jupyter notebook:

[gdmcbain]

How does this compare with meshio.svg?

[kinnala]

Some points:

• This outputs str whereas meshio.svg seems to write to file; this is needed in Jupyter object integration, see https://ipython.readthedocs.io/en/stable/config/integrating.html.
• This supports high-order meshes, curved boundaries, etc. (this will be more thorough when Reimplementation of skfem.mesh #556 is ready.)
• I plan to extend this so that we have skfem.visuals.svg.plot for visualizing solutions.

[gdmcbain]

Thanks.

I didn't know about the rich display feature of notebooks; that is an enticing feature. I wonder whether _repr_javascript_ would extend that to something like femgl for three-dimensional meshes.

[gdmcbain]

It might be possible to factor meshio.svg.write (over there) to construct the string and then write that string to a file, as we did here in skfem.io.meshio, but I see that that's partly hindered by mesho.svg.write using xml.etree.ElementTree.write which writes to ‘a file name, or a file object opened for writing’, although perhaps the latter could be an io.StringIO.

However, extension to ‘high-order meshes, curved boundaries, etc.’ I would think rather beyond meshio; I don't think there's enough common ground to support these features between mesh file formats.

Thanks.