[mixture.py] link create and recover functions by ".. seealso::" in docstrings
[r_value] add function "make_r_tmix"

o introduce private function "_make_r_patches" --> groups chains and
  partitions them, returns mean and covariance of each patch
o "make_r_gaussmix" and "make_r_tmix" call "_make_r_patches" and
  "create_gaussian_mixture" / "create_t_mixture" respectively.