Merge branch 'release/v2.0.0'
fixed generated code, updated the call to finite_diff_grad from src/stan/model/util.hpp, added unit test for finite_diff_grad
changed the test_grad call in util to match the test_grad call in v1.3
Merge pull request #302 from stan-dev/hotfix/issue-295-diagnose_not_working

Fixes #295.
Merge branch 'hotfix/v2.0.1' of https://github.com/stan-dev/stan into feature/parser_error_reporting
Hotfix merge, switched back to clang++