Feature/new recursion

[CosmoMatt]

@jasonmcewen sorry this is a rather enormous PR. Basically it

Core additions

• Adds JAX versions of all the lower level functions (e.g. resampling, quadrature etc).
• Develops on the fly (OTF) recursions that perform well on GPUs
• Integrates OTF recursions for full spherical harmonic and Wigner transforms
• Update both recursive and precompute transforms to get 64bit precision
• Pull out jax inverse FFTs (which are broken for 64bit precision)
• Wigner transforms are wrapped in lax.fori_loop() to reduce compile time
• Single-program multiple-data (SPMD) functionality added for all transforms for on-node GPUs. These are somewhat experimental APIs
• Benchmark all of the above for paper plots etc.

Following this there were a host of additional changes

• Overhauled the package directory structure
• Overhauled the GitHub readme and documentation
• Linked documentation to new code structure
• Added notebooks for spherical and Wigner transforms
• Address merge conflicts

and there are probably quite a few things that haven't been mentioned. What is left to do is

• Increase code coverage (if needed)
• Make repo public
• Set up google collab notebooks and add linked badges to readme etc.
• Deploy on PyPi

Then we will be in an alpha release so doubtless there will be some issues that come up as people (hopefully) begin to pick-up the code.