vtk: add typing to entire module

[alexfikl]

In my quest to add support for that HyperTreeGrid thing, I started slowing adding typing to the vtk module as well. It should be fairly complete now, but it has some actual changes in there to make mypy happy:

• Moved the copy from XMLElementBase to XMLElement (doesn't seem to be used anyway).
• Made the EncodedBuffer an ABC (it looked like it wanted to be a Protocol, but this seemed easier)
• Added a base class Visitable for DataArray, UnstructuredGrid and StructuredGrid to type "things that can generate XML".

The commits should be pretty independent, so could look at them separately. There may be some fishy things in there, but mypy --strict is happy at least!

[alexfikl]

Hm, this removed a space somewhere and it fails some meshmode test. I'll look at it tomorrow :(

EDIT: Found it! Should be ok now 🚀
EDIT2: Added a little test for that in here too (just doing a filecmp with the files that were already generated).

[inducer]

The commit message 'test: check reference output " suggest that output is being checked against a reference (which seems to have been added). I agree that that's a good idea, but I can't find the code that's doing the checking...?

[alexfikl]

The commit message 'test: check reference output " suggest that output is being checked against a reference (which seems to have been added). I agree that that's a good idea, but I can't find the code that's doing the checking...?

@inducer Oops, managed to remove that while rebasing. Should be in https://github.com/inducer/pyvisfile/compare/4d5c8163908a65f6654e8c0d8eed7bf05ea7a5d1..6592dbfa8b3221194e23848d5f3dc800895a86c6

[inducer]

Thanks!