sage.manifolds: Connect to Pymanopt

[mkoeppe]

​https://www.pymanopt.org/ provides numerical optimization algorithms over various families of manifolds (Sphere, Stiefel, Grassmann, ...) and their products.

I propose to

• add these manifolds as Sage classes
• each providing a method _manopt_ that produces the corresponding pymanopt class
• the default implementation of _manopt_ would create an instance of a subclass of ​https://www.pymanopt.org/docs/api-reference.html#pymanopt.manifolds.manifold.Manifold that implements the required methods using SageManifold's symbolic computation.

[mkoeppe]

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.