Expose eigenvectors and eigenvalues functions for real matrices

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] If a user-facing facing change was made, the documentation PR is here: https://github.com/stan-dev/docs/pull/527

Release notes

Added eigenvectors and eigenvalues functions which return the complex-valued eigenvectors or eigenvalues of a real (asymmetrix) matrix.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[WardBrian]

Note: just looking at the C++ for these functions in the math library, I believe this will fail expression tests. These functions are not templated

[WardBrian]

Currently running this against https://github.com/stan-dev/math/pull/2728. It would not be ready to merge until that is accepted