Template prim sum's return type on eigen matrices (code cleanup #987)

[cqfd]

Summary

This fixes a little bug that @bbbales2 noted in #987: prim's implementation of sum for Eigen::Matrix was always returning double, rather than its argument's (templated) element type. Per our discussion in the issue, I've also replaced sum's implementation for std::vector with the standard library algorithm std::accumulate.

Tests

One new test that makes sure that sum has the right return type for various argument types. Any other return cases I should check?

Side Effects

None.

Checklist

• prim implementation of sum has wrong return type #987

• Copyright holder: Alan O'Donnell

  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)
  • 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

[bbbales2]

Looks good to me

[bbbales2]

Thanks @cqfd!

[bbbales2]

Oh whoops, @cqfd, could you add a copyright holder to the pull req.?

[cqfd]

Whoops—just added it (that look right?).