Improvements to checkpointing for emcee/ptemcee

A number of improvements to the way checkpointing is handled for emcee and ptemcee

• Adds checkpointing for ptemcee
• Checkpoints by writing the sampler object to a pickle
• Only checkpoint on an appropriate signal
• Standardize method between emcee/ptemcee
• Also writes a chain.dat file of the zero-temperature walker positions
• Checkpoints now pick-up from where the run left-off

Note - this adds dill as a dependency, which is required to properly store the sampler object. dill is an improved version of the python pickle module, available on pypi and widely tested.

Closes #347 (closed)