[density] copy covariance exactly once; make sure data type is ``double``
[importance_sampling] add function ``combine_weights``

o ``combine_weights`` calculates the `deterministic mixture weights`
  given importance weighted samples and their proposals
[r_value] allow passing just variance instead of full covariance matrix

o functions ``make_r_gaussmix`` and ``make_r_tmix`` do no more calculate
  the full chain covariances but only the variance (covariance diagonals)
  using numpy.var