Display and write actual random seed value

[rok-cesnovar]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

Fixes #803 by overwriting the default random seed with the actual pseudo-random value that is used. There are no changes for when the random seed is not -1.

This also adds a test that will prevent from this change going unnoticed again.

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/)

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	4.97	4.92	1.01	1.04% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.98	-1.84% slower
eight_schools/eight_schools.stan	0.09	0.09	1.04	4.28% faster
gp_regr/gp_regr.stan	0.22	0.22	0.98	-1.94% slower
irt_2pl/irt_2pl.stan	6.06	6.05	1.0	0.13% faster
performance.compilation	88.26	87.18	1.01	1.22% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	7.39	7.48	0.99	-1.1% slower
pkpd/one_comp_mm_elim_abs.stan	21.72	20.41	1.06	6.03% faster
sir/sir.stan	93.3	98.34	0.95	-5.4% slower
gp_regr/gen_gp_data.stan	0.05	0.05	0.99	-1.33% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.04	3.0	1.01	1.13% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.32	0.32	1.0	-0.47% slower
arK/arK.stan	1.74	1.75	1.0	-0.25% slower
arma/arma.stan	0.74	0.8	0.92	-8.24% slower
garch/garch.stan	0.63	0.58	1.08	7.23% faster
Mean result: 1.00178439898

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Jenkins Console Log
Blue Ocean
Commit hash: 1c5c4d6

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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

[mitzimorris]

lgtm!