Consider moving high-level kernels (aliases) to submodules

[rlouf]

kernel.py is growing very large and the size is only ever going to increase not only as we add new algorithms but also as we add documentation. The fact that we need to use separators in the file is a good indicator that it is time to break this file down.

Aliases can be defined in __init__.py to simplify access.

We should also consider moving the adaptation module to mcmc.