- remove passing by const reference elementary types
- change `Jsum_dx` by `Jint_dx`
- control bounds now depend on whether they are defined
- remove unnecessary scopes
-change `2e19` by `std::numeric_limits`
- overwrite th_stop function wrapping `tol` parameter
- add preprocessor IPOPT guard where necessary
- fix unittest to not consider IPOPT if not installed
- add license in the files