Merge branch 'develop' of https://github.com/stan-dev/math into feature/operands_partials_less_copies

Merge branch 'feature/operands_partials_less_copies' of https://github.com/stan-dev/math into feature/operands_partials_less_copies

Change how users access OpenCL kernels

[WIP] [WIP] [WIP] a way of kerneling

Changes all the kernel files so that they are placed into const char*'s within each kernel file. Doxygen works but the object holding the kernel code is undocumented. All of the kernel structs are moved into the respective kernel file

change rev inclue to prim include

Add throw docs to add_diag and added tests to add_diag to verify behavior with 1 length vectors and 0 by 0 matrices (Issue #887)

Feature/issue 977 some variadic templating

make MPI build more robust... now outdated system installs of boost won't stop this to work.

Merge pull request #975 from roualdes/feature/issue-914-beta-distribution-mean-count-parameterization

feature/issue-914 Beta Distribution mean/precision parameterization

functions through kernel_cl.func(inputs).

Merge branch 'develop' of https://github.com/stan-dev/math into feature/issue-914-beta-distribution-mean-count-parameterization

Feature/GLMs Accept Scalars 

Merge branch 'feature/issue-914-beta-distribution-mean-count-parameterization' of https://github.com/roualdes/math into feature/issue-914-beta-distribution-mean-count-parameterization

Merge branch 'develop' of https://github.com/stan-dev/math into feature/issue-914-beta-distribution-mean-count-parameterization