Fixup code to remove goto methods, use vectors instead of arrays, and started breaking up large functions into smaller methods

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

Merge branch 'code-cleanup/return_types' of github.com:bstatcomp/math into code-cleanup/return_types