bugfix arena_matrix_cl to matrix_cl conversions

[t4c1]

Summary

Fixes a number of bugs related to conversions of arena_matrix_cl into matrix_cl.

Tests

Added tests for conversions and some more operations on arena_matrix_cl. Tests check that source and result of conversion have the same underlying cl::Buffer.

Side Effects

None.

Release notes

OpenCL: Fixed a number of bugs related to conversions of arena_matrix_cl into matrix_cl. Before this PR such a conversion invoked kernel generator and made a copy of data. Now only references to data are updated.

Checklist

• Math issue #(issue number)

• 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

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.11	3.1	1.01	0.62% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	1.0	0.27% faster
eight_schools/eight_schools.stan	0.11	0.13	0.87	-14.89% slower
gp_regr/gp_regr.stan	0.16	0.16	0.96	-3.68% slower
irt_2pl/irt_2pl.stan	5.9	5.93	0.99	-0.57% slower
performance.compilation	89.44	86.77	1.03	2.99% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.68	9.04	0.96	-4.18% slower
pkpd/one_comp_mm_elim_abs.stan	29.41	29.16	1.01	0.86% faster
sir/sir.stan	134.5	124.11	1.08	7.72% faster
gp_regr/gen_gp_data.stan	0.04	0.04	1.01	0.74% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.0	2.99	1.0	0.46% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.39	0.39	0.98	-1.61% slower
arK/arK.stan	2.53	2.54	1.0	-0.18% slower
arma/arma.stan	0.65	0.65	1.0	0.03% faster
garch/garch.stan	0.64	0.63	1.0	0.37% faster
Mean result: 0.994638708105

---

Jenkins Console Log
Blue Ocean
Commit hash: b8e60fd

---

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