Introducing SamplerConfig

[avibryant]

This is another try at refactoring Sampler to allow for more modular tuning. The approach here centers around a new SamplerConfig trait:

trait SamplerConfig {
  def iterations: Int
  def warmupIterations: Int

  def stepSizeTuner(): StepSizeTuner
  def metricTuner(): MetricTuner
  def sampler(): Sampler
}

The notable thing here is that it separately/orthogonally creates stateful Tuner and Sampler objects for each chain. There's now a Driver which takes care of threading things together, and Sampler's job is narrowed to just performing a single iteration of either the warmup or sampling phase, given a stepsize and metric. (Windowed tuning can be implemented within the tuners, and there's a mechanism for communicating a reset to the stepsize tuner when the metric gets replaced).

A backwards-compatible HMC() constructor is provided for an appropriate config. For the moment, EHMC and mass matrix tuning have been dropped to keep the PR small, and will be reintroduced.

[avibryant]

Ok, looks like I need to bring back EHMC so that the docs compile, before merging.