adds is_vt_complex and is_not_vt_complex type traits

[SteveBronder]

Summary

Adds two type traits is_vt_complex and is_vt_not_complex that checks whether the value type of a type T is complex or not. This will mostly be used in the stanc compiler when generating C++ functions from user defined functions.

Tests

Tests added to test both traits can be run with

./runTests.py test/unit/math/mix/meta/is_complex_test.cpp

Side Effects

Nope

Release notes

Adds type traits for detecting whether a type has a value type that is complex

Checklist

• Math issue

• 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/)

• 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

[WardBrian]

As an elaboration on what this allows, the following comparison:

WardBrian/stanc3@complex-containers...WardBrian:requires_all

This allows us to change a chain of stan::require_...s into one big require_all_t

[SteveBronder]

@rok-cesnovar would you mind taking a look at this?

[rok-cesnovar]

Cool beans!