Kernel refactor

I thought it would be nice to take the BlackJax approach, and be explicit about the kernel as a function of type: ``` MCMCState -> (MCMCState, MCMCInfo) ``` More concretely, I have introduced: ```python class MCLMCState(NamedTuple): """State of the MCLMC algorithm. """ x: ArrayTree u: ArrayTree l: float g: ArrayTree key : Any class MCLMCInfo(NamedTuple): transformed_x: ArrayTree l: ArrayTree de: float ``` I've only refactored for `sample_normal`, but it could easily be extended everywhere. This should make adding to BlackJax easier, but I think also clarifies the information flow of the code somewhat.