WIP: Making each sampler into its own library

Each sampler is in the process of being converted into its own library.
This commit does not reflect the new changes and probably does not run.
WIP: modularization of sampler code

The sampler specific details of the nested sampler code have been moved
into other functions to more clearly explain what they do.

MH and clustered are in the process of being tested.
Metropolis Hastings works

Metropolis hastings now works.
The clustered sampling family of samplers is currently broken.
Making progress on clustered sampler

Clustered sampler currently does not work,
it is still WIP
Sampler code has been modularized

Sampler code has been broken into smaller parts
Merge pull request #17 from rzlee/refactor

Refactor of samplers code