refactored init_adapt function to call a simpler init_adapt function that can be called through tests
adding a test that fails for init_adapt<>
fixed code to take in the appropraite starting value for init_adapt
Merge branch 'develop' into feature/issue-449-fix-initialization
merging develop. changing std::vector to Eigen::VectorXd
Merge pull request #450 from stan-dev/feature/issue-449-fix-initialization

Fixes #450.