update rep_matrix to have Ret typename template for var<VectorXd> case

[SteveBronder]

Summary

Just a small bugfix. Missed the typename Ret template parameter for rep_matrix when the input is a vector or row vector

Tests

No new tests

Side Effects

None

Release notes

Checklist

• Math issue How to add static matrix? #1805

• Copyright holder: Steve Bronder

  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	3.46	3.45	1.0	0.34% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.99	-0.52% slower
eight_schools/eight_schools.stan	0.09	0.09	0.99	-0.52% slower
gp_regr/gp_regr.stan	0.14	0.14	0.99	-1.5% slower
irt_2pl/irt_2pl.stan	5.09	5.1	1.0	-0.22% slower
performance.compilation	91.65	89.04	1.03	2.84% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.28	8.21	1.01	0.92% faster
pkpd/one_comp_mm_elim_abs.stan	28.73	29.73	0.97	-3.47% slower
sir/sir.stan	119.92	118.83	1.01	0.91% faster
gp_regr/gen_gp_data.stan	0.03	0.03	0.99	-1.14% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.05	3.0	1.02	1.64% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.37	1.04	3.92% faster
arK/arK.stan	2.04	2.04	1.0	0.45% faster
arma/arma.stan	0.27	0.27	1.0	-0.37% slower
garch/garch.stan	0.65	0.65	1.01	1.23% faster
Mean result: 1.00333325733

---

Jenkins Console Log
Blue Ocean
Commit hash: bba6132

---

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

[andrjohns]

@SteveBronder just merged in develop to re-trigger the CI build, will approve once all passes

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.6	3.54	1.02	1.81% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.97	-2.61% slower
eight_schools/eight_schools.stan	0.09	0.09	1.01	0.92% faster
gp_regr/gp_regr.stan	0.14	0.14	0.98	-1.7% slower
irt_2pl/irt_2pl.stan	5.8	5.74	1.01	1.02% faster
performance.compilation	93.78	90.76	1.03	3.22% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.1	8.04	1.01	0.82% faster
pkpd/one_comp_mm_elim_abs.stan	31.36	31.01	1.01	1.1% faster
sir/sir.stan	117.33	121.21	0.97	-3.3% slower
gp_regr/gen_gp_data.stan	0.04	0.04	1.01	0.5% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.99	3.03	0.99	-1.47% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.4	0.39	1.02	2.04% faster
arK/arK.stan	2.06	2.05	1.01	0.66% faster
arma/arma.stan	0.23	0.23	1.01	0.78% faster
garch/garch.stan	0.58	0.58	1.0	-0.32% slower
Mean result: 1.00261297636

---

Jenkins Console Log
Blue Ocean
Commit hash: 51e053e

---

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

[andrjohns]

LGTM

[SteveBronder]

@andrjohns thank you!!!