Add HarmonicDifferentials

[saraedum]

This is an experiment to compute harmonic differentials in a translation surface; there are lots of convergence problems at the moment and we might end up doing things very differently.

TODO

• We need parts of Add deformations and treat pyflatsurf surfaces like native surfaces #211 to compare differentials on differently triangulated surfaces.
• On the octagon: Compare quality of differential with the explicitly computed differential.
  • We need to compute the same differential in another triangulation so we to be able to map cohomology classes through deformations
  • To compare the differentials, we just need to evaluate at points which means we need to be able to map points through deformations.
• Given a differential, compute the location of its zeros and their order of vanishing. (Up to some numerical error to decide whether two zeros are the same or not.)
• For an L, construct the 1-form associated to any real class in the symplectic orthogonal of span(Re(ω), Im(ω)). Check that it has a double zero.
• Given a 1-form η, compute -1/2i int η wedge bar{η}, see _area_upper_bound() in the old optimization approach.
• On H_1 the algebraic intersection is a symplectic form. Its g-th exterior power is a volume form. Compute the quantity "symplectic volume(ball of radius 1)". See https://sagemath.zulipchat.com/#narrow/stream/271193-flatsurf/topic/Harmonic.20Differentials/near/420236362

Checklist

• Added an entry in doc/news/.
• Added a test for this change.
• Added new .py files to the documentation in doc/geometry or doc/graphical.