moved indicator_factory into tools
clean up

o put importance sampler into its own module
o introduced tools.indicator (collects former indicator_facory and
  merge_function_with_indicator)
o moved pmc to module cluster
o in gaussian_pmc rename kwarg proposal --> density
o renamed MixtureProposal --> MixtureDensity
o renamed PmcProposal --> ProbabilityDensity
o renamed StudentTComponent --> StudentT
o renamed GaussianComponent --> Gauss
o refined toctree in documentation
[tools/History] robustification

o included method __len__()
o robustification of append() --> check for invalid ``new_points_len``
o ordered methods: __init__ first, others alphabetically
[tools/plot] included into sphinx documentation