More flexible subsystem structure

src/systems/diffeqs/odesystem.jl

@@ -81,7 +81,7 @@ struct ODESystem <: AbstractODESystem
     """
     The internal systems. These are required to have unique names.
     """
-    systems::Vector{ODESystem}
+    systems::Vector{Any}
     """
     The default values to use when initial conditions and/or
     parameters are not supplied in `ODEProblem`.

src/systems/diffeqs/sdesystem.jl

@@ -82,7 +82,7 @@ struct SDESystem <: AbstractODESystem
     """
     The internal systems. These are required to have unique names.
     """
-    systems::Vector{SDESystem}
+    systems::Vector{Any}
     """
     The default values to use when initial conditions and/or
     parameters are not supplied in `ODEProblem`.

src/systems/discrete_system/discrete_system.jl

@@ -44,7 +44,7 @@ struct DiscreteSystem <: AbstractTimeDependentSystem
     """
     The internal systems. These are required to have unique names.
     """
-    systems::Vector{DiscreteSystem}
+    systems::Vector{Any}
     """
     The default values to use when initial conditions and/or
     parameters are not supplied in `DiscreteProblem`.

src/systems/jumps/jumpsystem.jl

@@ -71,7 +71,7 @@ struct JumpSystem{U <: ArrayPartition} <: AbstractTimeDependentSystem
     """The name of the system."""
     name::Symbol
     """The internal systems. These are required to have unique names."""
-    systems::Vector{JumpSystem}
+    systems::Vector{Any}
     """
     The default values to use when initial conditions and/or
     parameters are not supplied in `ODEProblem`.

src/systems/nonlinear/nonlinearsystem.jl

@@ -46,7 +46,7 @@ struct NonlinearSystem <: AbstractTimeIndependentSystem
     """
     The internal systems. These are required to have unique names.
     """
-    systems::Vector{NonlinearSystem}
+    systems::Vector{Any}
     """
     The default values to use when initial conditions and/or
     parameters are not supplied in `ODEProblem`.

src/systems/optimization/constraints_system.jl

@@ -47,7 +47,7 @@ struct ConstraintsSystem <: AbstractTimeIndependentSystem
     """
     The internal systems. These are required to have unique names.
     """
-    systems::Vector{ConstraintsSystem}
+    systems::Vector{Any}
     """
     The default values to use when initial conditions and/or
     parameters are not supplied in `ODEProblem`.

src/systems/optimization/optimizationsystem.jl

@@ -38,7 +38,7 @@ struct OptimizationSystem <: AbstractOptimizationSystem
     """The name of the system."""
     name::Symbol
     """The internal systems. These are required to have unique names."""
-    systems::Vector{OptimizationSystem}
+    systems::Vector{Any}
     """
     The default values to use when initial guess and/or
     parameters are not supplied in `OptimizationProblem`.

src/systems/pde/pdesystem.jl

@@ -59,7 +59,7 @@ struct PDESystem <: ModelingToolkit.AbstractMultivariateSystem
     """
     The internal systems. These are required to have unique names.
     """
-    systems::Vector
+    systems::Vector{Any}
     """
     A vector of explicit symbolic expressions for the analytic solutions of each
     dependent variable. e.g. `analytic = [u(t, x) ~ a*sin(c*t) * cos(k*x)]`.