adds varmat for log_inv_logit

[SteveBronder]

Summary

Adds varmat for log_inv_logit

Tests

Tests added to test varmat and standard vectors of varmats to make sure the vectorization framework works.

Side Effects

Nope

Release notes

Allows log_inv_logit to use struct of arrays

Checklist

• [x] Math issue #2804

• [x] Copyright holder: Steve Bronder

  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/)

• [x] 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)

• [x] the code is written in idiomatic C++ and changes are documented in the doxygen

• [x] the new changes are tested

[syclik]

This PR fixes #2804. @rok-cesnovar wanted to be assigned to it.