Torch-harmonics

* Computation of associated Legendre polynomials
* changed algorithm to compute the associated Legendre polynomials for improved stability
* Improved Readme

* Vector Spherical Harmonic Transforms
* projects vector-valued fields onto the vector Spherical Harmonics
* supports computation of div and curl on the sphere
* New quadrature rules
* Clenshaw-Curtis quadrature rule
* Fejér quadrature rule
* Legendre-Gauss-Lobatto quadrature
* New notebooks
* complete with differentiable Shallow Water Solver
* notebook on quadrature and interpolation
* Unit tests
* Refactor of the API

* Renaming from torch_sht to torch_harmonics
* Adding distributed SHT support
* New logo

* Single GPU forward and backward transform
* Minimal code example and notebook