Add tuple-returning special functions

[WardBrian]

Summary

This adds the following functions which return tuples. Each is already implemented piecewise, these versions just reduce computation:

• qr_thin
• eigendecompose_sym
• eigendecompose
• complex_schur_decompose
• svd
• csr_extract

Tests

Existing tests for the base functions were copied and applied to these tuple-returning variants.

Side Effects

Release notes

Added new functions qr_thin, eigendecompose_sym, eigendecompose, complex_schur_decompose, svd, and csr_extract.
Each of these is equivalent to calling several existing functions, but should be more efficient due to sharing work. For example, svd(m) is equivalent to (svd_U(m), singular_values(m), svd_V(m)).

Checklist

• Math issue: Closes Add tuple-returning versions of special functions #2845

• Copyright holder: Simons Foundation

  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]

Looked over the code and looks good! Reviewing the tests now

[SteveBronder]

Few small comments

[SteveBronder]

One small comment but besides that lgtm!