Refine strategy interface

[kazewong]

Currently the type of "resources" available to flowMC is basically hard coded. There is a local sampler and there is a global sampler. I think we can further change the interface to be the following:

The sampler has two main components: resource and strategy. resource should be something like local_sampler or global_sampler, and strategy what it is now.

Strategies will basically take resources in, goes through its own sampling procedure, hold some summary internal and return some resource back. I think this helps the interface to be more general.