All multivariate distributions should be generic over dimension

[YeungOnion]

#209 changes our API for the multivariate distributions. We'll need to update the other uses of multivariate for consistency.

• update Dirichlet to nalgebra multivariate #254
• review relevant multivariate PR's
  • off the cuff I know about Add CDF for multivariate normal #178 and Multivariate students t distribution #176 -> multivariate students t distribution #266
• update multivariate related traits in statistics module' #255
• Wishart and InverseWishart are proposed in Adds Wishart and InverseWishart distributions #170

[YeungOnion]

Will make sure to mention #170 needs these updates as well. But I'll drop the item from the above list (edited initial post to express that) since all of what's released should consistently be generic over dimension.