[_tools.py]            corrected typo in docstrings
[indicator_factory.py] clarified docstings
[markov_chain.py]      clarified docstings
[_tools.py]   - added function "_add_to_docstring"
[proposal.py] - using the new _tool: better discrimination of
                requirements on ``rng`` according to the classes
              - removed "adapt" (will be moved to markov_chain)
[markov_chain.py] introduced class "AdaptiveMarkovChain"