[tools] correct ESS and indicator

o ESS:         do not mask zero weights
o indicator: - copy defining arrays (upper/lower; center)
             - bugfix in ball_indicator
[tools] Add integer partition and patch_data
[plot_mixture] always copy ``ellypse_style`` to avoid overwriting the input
[density] cythonize student_t.py, improve argument handling in Gauss and StudentT
[density] Add [create,recover]_t_mixture, improve doc and error handling of create_gaussian_mixture
[sampler] Speed up importance_sampling with multi_evaluate
[mix_adapt] Add r value grouping and patching including an example
[mix_adapt] Implement student's t PMC including dof adaptation
[doc] Update docstrings to reflect new implementation and knowledge
[setup.py] use ``numpy.get_include`` to get the correct include path