Adds const ref and ref returns for to_var/fvar methods

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Fixed matrix_cl copying and moving

Fixes #1221 Merge pull request #1303 from bstatcomp/feature/issue-1221-opencl-rev-mdivide-left-tri-v2

Feature/issue 1221 Use OpenCL in rev/mdivide_left_tri 

This reverts commit f13ca590.

# Conflicts:
#	stan/math/opencl/matrix_cl.hpp

Feature/issue 1221 Use OpenCL in rev/multiply

Bugfix/646 log_sum_exp and log_diff_exp boundaries

Fixes #1312 fix matrix_cl_view test

matrix_cl copy assigment now does a deep copy. Added move constructor, move assignment operator and fixed an event-related bug.

issue 1279 - Remove deprecated Eigen content from math headers

Re-apply #1278 OpenCL prim multiply

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