Improve support for semi-Riemannian manifolds

[mateuszbaran]

Semi-Riemannian manifolds require different methods, see https://arxiv.org/abs/1812.07643 .

• Finding ONBs,
• indefinite orthogonal group,
• pseudo-spheres,
• pseudo-hyperbolic spaces,
• semi-Riemannian geometry of SPD,
• semi-Riemannian geometry of matrix Lie groups (?)

[kellertuer]

Should we introduce an interims type AbstractRiemannianManifold <: AbstractManifold and a AbstractSemiRiemannianManifold <: AbstractManifold to distinguish between manifolds that are Riemannian and those that are not? Might be too complicated since it depends on the metric and then the MetricManifold-Decorator might actually change that?

[mateuszbaran]

I'd prefer to have a trait instead of an abstract type like AbstractRiemannianManifold but there should definitely be a way to distinguish Riemannian and pseudo-Riemannian manifolds.

[kellertuer]

A trait sounds good, because that means we can “activate” it for different metric manifolds.