Sgmcmcjax

- added more documentation
- added PyMC integration example

See [this milestone](https://github.com/jeremiecoullon/SGMCMCJax/milestone/1) for details.

This first release includes the main aspects of the library: some standard sgmcmc samplers and some documentation.

The code is structured in 3 levels of abstractions. These levels are 1. diffusion solvers, 2. transition kernels, and 3. sampling functions.