Add tuple overload and cleanup code for promote_scalar

[SteveBronder]

Summary

This adds an overload to promote_scalar for the compiler that allows the input to be a set of tuples and the promotion type to be a tuple of scalar types. So for the compiler impl of tuples we will be able to generate code such as

std::tuple<std::vector<var>, var> = 
  promote_scalar<std::tuple<var, var>(std::vector<double>{...}, double(2.0));

This also cleanups the code for promote_scalar a little bit. Instead of having to do some roundabouts using a static struct method for doing actually the promotions we called from promote_scalar we just have explicit free functions for the particular cases we need that can be called from one another.

Tests

Tests are added for testing promotions of tuples with various inner types and scalar types. A test is also added for the new type trait is_tuple<> which just detects if a template is a tuple or not.

./runTests.py ./test/unit/math/ -f promote_scalar_test
./runTests.py ./test/unit/math/ -f is_tuple

Side Effects

Nope

Release notes

Add tuple overload and cleanup code for promote_scalar

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]

Just to check, does this do the right thing if it is called with a std::vector of tuples?

E.g, given a
std::vector<std::tuple<int,int>> vector;
Does the call
promote_scalar<std::tuple<double, double>>(vector)

Work like it does when the vector contains non-tuples (element wise?)

[SteveBronder]

No it did not, good catch! But I just put in an update that fixes that with a test for it as well

[WardBrian]

I mentioned this in the assign PR, but it is worth testing/making sure we support at least the following:

a tuple
An array of tuples
A tuple which contains an array
A tuple which contains a tuple
A tuple which contains an array of tuples

The kind of recursive logic needed to get all that right usually generalizes to even deeper nesting

[SteveBronder]

I'll double check but looking at the test I believe I cover all of those

[rok-cesnovar]

@SteveBronder is this ready for review? I have been tagged for this a few weeks ago - just want to make sure.

[rok-cesnovar]

This looks good to me. I went ahead and merge recent develop in as the branch was pretty old at this point. Merge away assuming the tests now pass.