it was planned for use in multivariate operands and partials.

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.

http://discourse.mc-stan.org/t/sequence-view-design/261/2

moving stan/math/prim/scal/fun/inverse_softmax.hpp to arr; moving stan/math/prim/mat/err/check_matching_sizes.hpp to arr