consider renaming `con_outputs`

[yunlongxu-numagic]

Imagine we have some model outputs that we need to use in the objective function of the ocp

• we need to formulate objective function using these outputs
• then these outputs MUST be in the decision variables
• then we need to add these outputs to the con_outputs

but in this case, con_outputs is no longer constrained outputs, but outputs that need to be in the decision variables