Feature/variadic algebra solvers

[charlesm93]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here: (in progress)
  • [ ] OR, no user-facing changes were made

Release notes

Exposes algebraic functions with a variadic arguments, using the following four functions: solve_powell, solve_powell_tol, solve_newton and solve_newton_tol. Relevant changes to Stan-math are already merged.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[charlesm93]

@rok-cesnovar can you review this one?

[rok-cesnovar]

Sure thing. Can do this evening.