Add to_matrix_cl() vector<var> and minor OpenCL fixes

[rok-cesnovar]

Summary

This PR:

• adds to_matrix_cl for std::vector<var_value>
• fixes a header guard that caused rows(matrix_cl) to not be available
• reorder the main header file (its not opencl/prim.hpp and opencl/rev.hpp)

Tests

Added simple test for the new function.

Side Effects

/

Release notes

Added to_matrix_cl(std::vector<var_value>) and reorganized main Stan Math OpenCL headers.

Checklist

• Copyright holder: Rok Češnovar, Uni. of Ljubljana

  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/)

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.46	3.57	0.97	-3.1% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.97	-3.6% slower
eight_schools/eight_schools.stan	0.12	0.11	1.06	5.67% faster
gp_regr/gp_regr.stan	0.16	0.17	0.99	-1.18% slower
irt_2pl/irt_2pl.stan	5.98	5.79	1.03	3.12% faster
performance.compilation	86.96	85.86	1.01	1.27% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.45	8.42	1.0	0.38% faster
pkpd/one_comp_mm_elim_abs.stan	29.51	29.08	1.01	1.46% faster
sir/sir.stan	129.66	128.91	1.01	0.58% faster
gp_regr/gen_gp_data.stan	0.04	0.04	0.99	-0.65% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.07	2.92	1.05	5.16% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.38	0.99	-1.11% slower
arK/arK.stan	1.78	1.78	1.0	-0.17% slower
arma/arma.stan	0.76	0.73	1.04	3.8% faster
garch/garch.stan	0.63	0.61	1.02	2.03% faster
Mean result: 1.00991719912

---

Jenkins Console Log
Blue Ocean
Commit hash: 3ae5aa1

---

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

[rok-cesnovar]

How rude of you Jenkins :)

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.52	3.5	1.01	0.61% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.97	-3.02% slower
eight_schools/eight_schools.stan	0.12	0.11	1.02	2.02% faster
gp_regr/gp_regr.stan	0.17	0.16	1.01	0.74% faster
irt_2pl/irt_2pl.stan	5.73	5.79	0.99	-0.92% slower
performance.compilation	88.21	85.79	1.03	2.74% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.42	8.44	1.0	-0.26% slower
pkpd/one_comp_mm_elim_abs.stan	29.78	29.58	1.01	0.69% faster
sir/sir.stan	137.98	130.84	1.05	5.17% faster
gp_regr/gen_gp_data.stan	0.04	0.04	0.98	-2.14% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.95	2.93	1.01	0.6% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.38	1.0	0.35% faster
arK/arK.stan	1.78	1.78	1.0	-0.15% slower
arma/arma.stan	0.73	0.73	1.0	0.19% faster
garch/garch.stan	0.62	0.61	1.01	1.36% faster
Mean result: 1.005710785

---

Jenkins Console Log
Blue Ocean
Commit hash: cd23348

---

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