Refactoring Sampler and Progress

[avibryant]

This breaks Sampler into:

• Sampler, which is now concrete, and mostly just contains a list of Warmup steps
• SamplerState, which has all the mutable state, including what used to be in ProgressState, and also the internal config state of the sampler (eg stepSize); it wraps LeapFrog
• Warmup steps which get passed SamplerState and are expected to update it adaptively somehow

There are now warmup steps for the initial "find reasonable step size" phase, dual averaging, and the empirical path length distribution from EHMC. You can now construct a custom sampler by specifying the warmup schedule, ie,

Sampler(iterations)
   .findInitialStepSize
   .dualAverageStepSize(warmupIterations)
   .fixedPathLength(nSteps)