Geotorch

GeoTorch now is fully compatible with `torch.nn.utils.parametrize` and it uses it whenever possible.

More flexible initialisation
The current release implements a number of important changes
- Now it's possible to initialise a constraint to a fixed matrix by simply assigning to the constrained weight
- Complete rewrite of the logic that allowed to heavily simplify it
- Updated to be compatible with `torch.nn.utils.parametrize`, which will be part of PyTorch 1.9
- Added many new spaces
- More thorough testing. All the possible constructions are now tested for every manifold

Initial release

Implements the following manifolds:
- ``Rn(n)``: Rⁿ. Unrestricted optimization
- ``Sym(n)``: Vector space of symmetric matrices
- ``Skew(n)``: Vector space of skew-symmetric matrices
- ``Sphere(n)``: Sphere in Rⁿ. It is Sⁿ⁻¹ = { x ∈ Rⁿ | ||x|| = 1 }
- ``SO(n)``: Manifold of n×n orthogonal matrices
- ``Stiefel(n,k)``: Manifold of n×k matrices with orthonormal columns
- ``Grassmannian(n,k)``: Manifold of k-dimensional subspaces in Rⁿ
- ``LowRank(n,k,r)``: Variety of n×k matrices of rank r or less

And the following constructions:
- ``Manifold``: Manifold that supports Riemannian Gradient Descent and trivializations
- ``Fibration``: Fibred space π : E → M, constructed from a ``Manifold`` E, a submersion π and local sections of dπ
- ``ProductManifold``: M₁ × ... × Mₖ