Fix: eigenvalues returns column vector

[WardBrian]

Summary

Eigen's eigensolvers return column vectors, but the current return type is set to a row vector. This causes issues such as stan-dev/stan#3199

Tests

Side Effects

Release notes

Fixed an issue with the return type of eigenvalues being incorrect

Checklist

• Math issue: Closes complex_vector assigment matching-size check sometimes broken stan#3199

• Copyright holder: Simons Foundation

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  - 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

[syclik]

"Request changes" turned on so I have a little bit of time to look at how the return types could have gotten through.

[WardBrian]

@syclik - anything I can do to help?

[syclik]

Thanks! I'm approving and let's get this merged.

I don't think this PR needs to wait any longer. I still want to see how this is happening.

After this issue, I'm even more adamant on not using auto for return types except where there's a really good reason for it. Types really provide a lot of information for the developer.

[syclik]

LGTM! Thanks!!