Adjust signatures of eigenvalues and eigenvectors to accept expressions

[WardBrian]

Summary

This adjusts the signatures of the eigenvalues and eigenvectors functions to accept eigen expressions as input. This will allow them to be exposed in the Stan language: stan-dev/stanc3#1192

Tests

Existing tests for functionality remain in place. Additional expression tests will be automatically run when exposed in the compiler, I've run them locally for now.

Side Effects

None

Release notes

Updated the signatures of eigenvalues and eigenvectors to use our pseudo-concept require templates.

Checklist

• Math issue #(issue number)

• Copyright holder: Simons Foundation

  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

[bob-carpenter]

This is fine as is. My comments were more questions/optional and about style, not content.