Bugfix/646 log_sum_exp and log_diff_exp boundaries

Fixes #1312 fix matrix_cl_view test

issue 1279 - Remove deprecated Eigen content from math headers

Re-apply #1278 OpenCL prim multiply

mdivide_right_tri can use OpenCL

Fixes #1297 make key of map for opencl kernel options into a string

[Jenkins] auto-formatting by clang-format version 5.0.2-svn328729-1~exp1~20180509124008.99 (branches/release_50)

Fixes #1198 Added triangularity attribute to matrix_cl

Refactor for enable_if functions

[Jenkins] auto-formatting by clang-format version 5.0.2-svn328729-1~exp1~20180509124008.99 (branches/release_50)

# Conflicts:
#	stan/math/opencl/add.hpp
#	stan/math/opencl/cholesky_decompose.hpp
#	stan/math/opencl/copy.hpp
#	stan/math/opencl/copy_triangular.hpp
#	stan/math/opencl/err/check_diagonal_zeros.hpp
#	stan/math/opencl/err/check_nan.hpp
#	stan/math/opencl/err/check_symmetric.hpp
#	stan/math/opencl/kernel_cl.hpp
#	stan/math/opencl/matrix_cl.hpp

Changes all prim files to use *_return_type_t instead of typename *_return_type

opencl prim gp_exp_quad_cov