restructured usage of _Hist

o made submodule _tools partially public
o removed class _Chain
o renamed class _Hist -> History
o removed accept_count from classes _Hist/History
o History/_Hist now supports being empty
o classes MarkovChain, ImportanceSampler: renamed self.hist -> self.history
o class MarkovChain accept_count is returned by method run() but not saved
o class AdaptiveMarkovChain saves self._last_accept_count