Add deformations and treat pyflatsurf surfaces like native surfaces

[saraedum]

Here we bring a system like the libflatsurf Deformation maps to sage-flatsurf. The idea is that this allows us to map all kinds of objects between surfaces that are related to each other.

To make this fully functional, all operations that modify a surface need to produce a Deformation instead that can (sometimes) be used to map objects.

In particular this will allow us to understand whether subdivision for harmonic differentials actually converge correctly (currently, it's extremely annoying to relate a point on the surface before and after subdivision and retriangulation.)

At the same time, we introduce Surface_pyflatsurf which provides a pyflatsurf FlatTriangulation with the sage-flatsurf Surface interface. In particular, a libflatsurf Deformation then can be naturally turned into a Deformation_pyflatsurf to map sage-flatsurf objects across it.

Checklist

• [x] Added an entry in doc/news/.
• [x] Added a test for this change.
• [ ] Added new .py files to the documentation in doc/geometry or doc/graphical.
• [ ] Ensure that TestSuite runs for all new objects.

[saraedum]

Nothing has happened here yet and nothing can be played with yet.

[saraedum]

This won't be merged as is. We need to cherry-pick small bits out of it as we did in #295. We'll use #213 as a base to cherry-pick from into smaller PRs.