Clean up ddg.math.symmetric_matrices

Contributes to #518.

I just cleaned some smaller things up and improved the documentation. But some questions remain:

• At some point, there was the idea to move the Signature classes somewhere else, for example to the ddg.geometry.quadrics module. That particular module is not the best place imo, because the signature classes don't depend on quadrics in any way, only the other way around. But I still think we might want to move them in the future.
• The module mixes functionality for raw symmetric matrices and "projective" functionality for symmetric matrices up to nonzero scalar multiplication. We might want to separate this more clearly.

Both of these are tasks for the future imo.