[all but markov_chain_test.py] -improved documentation
[markov_chain_test.py]         -removed call to array-contructor;
                                do not need to copy the array
                                because it is not changed
[markov_chain] Update doc, add references
[indicator_factory] Remove default args
[classes _Chain, _Hist, MarkovChain] -moved _Chain.clear() --> _Hist.clear()
                                     -renamed prealloc_for --> prealloc
[markov_chain] -imporoved documentation
               -default covaariance scaling changed 1. --> 2.38**2/dimension
[cluster] First version of hierarchical clustering
[cluster] Simplify and bugfix hierarchical clustering

o fix prune()
o make comp. weights available as array
o simplify regroup()
o improve doc
o stricter tests
reconfigured git to ignore backup files
renamed module mcprerun --> markov_chain
[cluster/hierarchical] redefinitions for compatibility with python3
[_tools._chain._Hist] replaced get-functions by item-access []
[markov_chain] added example
[_Hist] resorganized item access (no change in usage)
[cluster] introduced variational clustering
[cluster/variational.py] - initialization of x_mean_comp = m0
                         - std-value for m0 is no more zeros(...)
                         - bugfix prune
                         - safe _update_r (avoid division by 0)
[cluster] Comments and todos in variational