Added unit tests that checks the symmetry of quad_form_sym
Enable quad_form_sym
add quad_form_sym unit test to validate asymmetry
removing use of std::vector at() method. using [] instead
adding a recorder that sums values. for use in RStan.
changing member variables to reference variables
adding includes to sum_values.hpp
Adding include for stdexcept.
quad_form_sym: minor changes in unit test
Merge pull request #651 from stan-dev/feature/sum_recorder

Feature/sum recorder
Merge pull request #645 from stan-dev/feature/issue-268-quad_form_sym

Fixes #268. Feature/issue 268 quad form sym
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-567-manual-2.3.0
updated manual with latest suggestions