Add missing unary plus and cleanup tests

[rok-cesnovar]

Summary

Fixes #1888 by adding the unary plus. When adding tests for that I saw that we still have the _works_with_other_functions tests that are redundant after #1980 This advances #1886 but does not close it yet.

Tests

Added same tests as for minus()

Side Effects

/

Release notes

Added missing implementations for the unary plus.

Checklist

• Math issue No implementation of plus(x) #1888

• Copyright holder: Rok Češnovar, Univ. 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/)

• 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

[t4c1]

Use perfect forwarding when returning the argument.

[rok-cesnovar]

Like this?

template <typename T>
inline plain_type_t<T> plus(T&& x) {
  return std::forward(x);
}

[t4c1]

Yes, also make the return type T - we don't need plus to eval its argument.

[rok-cesnovar]

@serban-nicusor-toptal can we try if gelman-group-linux can now run the Clang-format and Linting & Doc checks again? If we are unlucky with Linux AWS instances like we are now and there are none free, the jobs are all stalled.

If there is an issue with the machine, we can see if we can fix it.

[serban-nicusor-toptal]

Hey @rok-cesnovar the pool was enough, there were no issues with it not being able to find machines.
I either introduced a bug yesterday or there were some connectivity issues today, I've recreated the fleets now and everything is working fine.
Sorry for the waiting!
PS: I think gelman-linux should run everything just fine now.

[rok-cesnovar]

@serban-nicusor-toptal thanks!

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	4.19	4.23	0.99	-1.02% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	1.0	0.04% faster
eight_schools/eight_schools.stan	0.09	0.09	0.98	-2.13% slower
gp_regr/gp_regr.stan	0.2	0.2	0.99	-1.02% slower
irt_2pl/irt_2pl.stan	5.25	5.28	0.99	-0.57% slower
performance.compilation	88.81	87.58	1.01	1.38% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.27	8.21	1.01	0.7% faster
pkpd/one_comp_mm_elim_abs.stan	27.02	27.11	1.0	-0.34% slower
sir/sir.stan	129.33	130.82	0.99	-1.15% slower
gp_regr/gen_gp_data.stan	0.05	0.05	0.99	-1.16% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.34	3.33	1.0	0.15% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.4	0.94	-6.39% slower
arK/arK.stan	1.84	2.54	0.72	-38.47% slower
arma/arma.stan	0.59	0.73	0.8	-24.3% slower
garch/garch.stan	0.62	0.73	0.84	-18.63% slower
Mean result: 0.95064261234

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Jenkins Console Log
Blue Ocean
Commit hash: a74a277

---

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