fix template error for matrix fma

[SteveBronder]

Summary

Bug fix for stan-dev/stanc3#1150 where fma would choose the wrong template because one was too broad.

Tests

static assert added to scalar/matrix combinations of fma in tests to make sure correct return type is given.

Side Effects

Nope

Release notes

Replace this text with a short note on what will change if this pull request is merged in which case this will be included in the release notes.

Checklist

• Math issue stanc issue

• Copyright holder: Steve Bronder

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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