student_t_qf only returns column eigen vectors now

[SteveBronder]

Summary

Fixes the compile issue in stan-dev/stanc3#1620 . The code was trying to be a bit too clever with it's return type. Now, no matter what shape the input containers are, we will return a Eigen column vector. I think having the return type be dependent on the container shape gets kind of hard. I'm not sure how to handle cases where you mix colums, row, and std::vector types as containers. So always returning a column vector is just simpler.

Tests

Just flipped one test to use the eigen column vector output instead of row vector output.

Checklist

• 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