Expose additional complex signatures

[WardBrian]

This depends on https://github.com/stan-dev/math/pull/2753 and https://github.com/stan-dev/math/pull/2754

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/549

Release notes

Added signatures for addition, subtraction and division which mix scalars and complex vector types, broadcasting the scalar as appropriate.

Added svd_U, svd_V, singular_values, eigenvalues_sym, and eigenvectors_sym for complex inputs

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]

@rok-cesnovar - just added more signatures for elt_multiply and divide, re-review?

[rok-cesnovar]

All good!