This interface allows users to add secondary differential equations to a primary set of differential equations.
In some cases users may need to integrate some problem-specific equations along with a primary set of differential equations. One example is optimal control where adjoined parameters linked to the minimized hamiltonian must be integrated.
This interface allows users to add such equations to a primary set of {@link FirstOrderDifferentialEquations first order differential equations}thanks to the {@link ExpandableStatefulODE#addSecondaryEquations(SecondaryEquations)}method.
@see ExpandableStatefulODE
@since 3.0