Consistent treatment of frequency reporters

[wiederm]

Currently, we save the parameter that controls the frequency reporters are called in the Sampler class.
We pass it in its __init__ method --- should we move this to the run method where we also set the number of sampling steps?

Also, we are currently saving coordinate sets for debugging in the sampler, should we move this to the reporter (this would only require to not write the buffer to disk but keep it in memory).

Originally posted by @chrisiacovella in #18 (comment)