We weren't getting expected behavior from some of these tests (e.g.
return_type<std::vector<matrix_v>> should be var, not matrix_v). I think
I made them simpler and they give me expected behavior for all the
use-cases I'm aware of.

Use cumulative_sum instead of nested loop in categorical_rng

small typo on PULL_REQUEST_TEMPLATE.md

Feature/issue 492 promotor fun refactor

cov_exp_quad error messages

Feature/issue 384 blocked cholesky

vector_seq_view refactor - replaces VectorViewMvt

This reverts commit 9c58974a.