Add Jacobian field to StochSystem

[oameye]

In many algorithms one needs the jacobian of the drift field. These are now computed using ForwardDiff. Adding the the field jacobian to StochSystem, makes it possible for the user to hardcode it or compute it symbolically using, e.g., ModeligToolkit.

[reykboerner]

Agreed, integration of ModelingToolkit and the possibility to hardcode the Jacobian of the drift should be included.