Runtime OpenCL device selection, allow seed=0 and fix allow-undefined user_header bug

[rok-cesnovar]

Summary:

Fixes one part of #825
Fixes #941
Fixes #953

The main part of this PR is allowing runtime selection of OpenCL devices.

./examples/bernoulli/bernoulli sample data file=examples/bernoulli/bernoulli.data.json opencl platform=0 device=0

Specifying only platform or only device IDs will print

Please set both device and platform OpenCL IDs.

Using opencl args when the model was not compiled with STAN_OPENCL will print

opencl is either mistyped or misplaced.
Re-compile the model with STAN_OPENCL to use OpenCL CmdStan arguments.
Failed to parse arguments, terminating Stan

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Rok Češnovar

By submitting this pull request, the copyright holder is agreeing to 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/)

[rok-cesnovar]

This is a minor fix for #953

[rok-cesnovar]

The changes to this file is a fix for #941

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.45	3.58	0.96	-3.68% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	1.0	-0.22% slower
eight_schools/eight_schools.stan	0.12	0.12	1.0	-0.25% slower
gp_regr/gp_regr.stan	0.17	0.16	1.02	1.69% faster
irt_2pl/irt_2pl.stan	5.79	5.78	1.0	0.13% faster
performance.compilation	88.51	85.87	1.03	2.98% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.4	8.45	0.99	-0.52% slower
pkpd/one_comp_mm_elim_abs.stan	30.24	29.08	1.04	3.85% faster
sir/sir.stan	131.93	133.87	0.99	-1.47% slower
gp_regr/gen_gp_data.stan	0.04	0.04	0.99	-0.53% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.93	2.97	0.99	-1.27% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.4	0.96	-3.97% slower
arK/arK.stan	1.77	1.77	1.0	-0.26% slower
arma/arma.stan	0.74	0.59	1.25	19.68% faster
garch/garch.stan	0.61	0.59	1.03	3.01% faster
Mean result: 1.01646923088

---

Jenkins Console Log
Blue Ocean
Commit hash: 9b291e7

---

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

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.44	3.47	0.99	-0.87% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.98	-2.11% slower
eight_schools/eight_schools.stan	0.12	0.12	0.99	-0.66% slower
gp_regr/gp_regr.stan	0.16	0.16	1.0	-0.38% slower
irt_2pl/irt_2pl.stan	5.8	5.82	1.0	-0.35% slower
performance.compilation	88.18	85.9	1.03	2.59% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.43	8.56	0.99	-1.52% slower
pkpd/one_comp_mm_elim_abs.stan	29.83	31.28	0.95	-4.84% slower
sir/sir.stan	131.88	129.41	1.02	1.88% faster
gp_regr/gen_gp_data.stan	0.04	0.04	1.0	0.21% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.94	2.94	1.0	-0.16% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.39	0.98	-2.46% slower
arK/arK.stan	1.79	1.75	1.02	1.95% faster
arma/arma.stan	0.75	0.59	1.27	21.49% faster
garch/garch.stan	0.61	0.59	1.03	3.1% faster
Mean result: 1.01622961698

---

Jenkins Console Log
Blue Ocean
Commit hash: 54ae958

---

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

[SteveBronder]

lgtm!