Add OpenCL prod

[t4c1]

Summary

Added OpenCL implementation for prod function and kernel generator operation for rowwise, colwise and 2d product.

Tests

New functions are tested.

Side Effects

None.

Release notes

Added OpenCL implementation for prod function and kernel generator operation for rowwise, colwise and 2d product.

Checklist

• Math issue Implement matrix_cl overloads for prim & rev functions #1854

• Copyright holder: Tadej Ciglarič

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[rok-cesnovar]

LGTM, just a bit of grammar nitpicking.

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.41	3.43	0.99	-0.64% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.99	-1.27% slower
eight_schools/eight_schools.stan	0.12	0.11	1.03	3.06% faster
gp_regr/gp_regr.stan	0.16	0.16	0.97	-3.31% slower
irt_2pl/irt_2pl.stan	5.39	5.26	1.02	2.29% faster
performance.compilation	90.32	88.34	1.02	2.19% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.91	8.88	1.0	0.32% faster
pkpd/one_comp_mm_elim_abs.stan	30.06	30.47	0.99	-1.38% slower
sir/sir.stan	134.55	129.81	1.04	3.52% faster
gp_regr/gen_gp_data.stan	0.04	0.04	0.98	-2.46% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.08	3.14	0.98	-1.86% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.37	1.02	1.56% faster
arK/arK.stan	1.89	1.88	1.0	0.43% faster
arma/arma.stan	0.64	0.65	1.0	-0.37% slower
garch/garch.stan	0.53	0.51	1.03	3.02% faster
Mean result: 1.00386017622

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Jenkins Console Log
Blue Ocean
Commit hash: 9625be6

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Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix