Fix Simulator blocking non-zero D (#398)

* Remove assertion in Simulator blocking non-zero D

* Update src/simulators.jl

Co-authored-by: Fredrik Bagge Carlson <baggepinnen@gmail.com>

* Update y in Simulator type

Co-authored-by: Fredrik Bagge Carlson <baggepinnen@gmail.com>

Author: albheim

src/simulators.jl

@@ -38,10 +38,9 @@ plot(t, s.y(sol, t)[:], lab="Open loop step response")
 """
 function Simulator(P::AbstractStateSpace, u::F = (x,t) -> 0) where F
     @assert iscontinuous(P) "Simulator only supports continuous-time system. See function `lsim` for simulation of discrete-time systems."
-    @assert all(P.D .== 0) "Can not simulate systems with direct term D != 0"
     f = (dx,x,p,t) -> dx .= P.A*x .+ P.B*u(x,t)
-    y(x,t) = P.C*x #.+ P.D*u(x,t)
-    y(sol::ODESolution,t) = P.C*sol(t)
+    y(x,t) = P.C*x .+ P.D*u(x,t)
+    y(sol::ODESolution,t) = P.C*sol(t) .+ P.D*u(sol(t),t)
     Simulator(P, f, y)
 end