We change the solver_options API by removing the workaround for IPOPT to
pass it options. This behavior changed between CasADi 2 and 3. The
workaround makes using solver other than IPOPT harder however, so instead
we go back to the old way of just passing through most options directly to
CasADi.

This commit alo introduces a new option "casadi_solver" for the dictionary
returned by solver_options(). The default value is "nlpsol" for
backwards compatibility. Another typical option would be "qpsol".

It also introduces a new API function "solver_success()" to override
what solver return status should be considered succesful, and how to
log said status.

Closes #1006