Vectorise binary log_sum_exp

[andrjohns]

Summary

This is a small PR vectorising the binary signature of log_sum_exp, which was missed when vectorising all binary functions

Tests

Additional prim and mix tests added for the broadcasted versions

Side Effects

N/A

Release notes

Vectorised binary signature for log_sum_exp

Checklist

• Math issue vectorize binary log_sum_exp #2924

• Copyright holder: Andrew Johnson

  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

[SteveBronder]

lgtm!