Add support for multivariate samples and distributions

[jacobaustin123]

To do:

• Add support for a multivariate Vector/Matrix class that can be contained inside variables with appropriate overloads.
• Add support for basic multivariate distributions, hopefully as overloads of existing distributions
• Add vector operations like matrix multiplication and dot products.

Design ideas:

ppl::Data<double> var {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
ppl::Param<double> param {{1, 2, 3}};
ppl:DataView<3, 4> view(double * ptr);