feat: add an always-present mutable cache key to the system

Author: AayushSabharwal

src/systems/abstractsystem.jl

@@ -805,7 +805,9 @@ has_equations(::AbstractSystem) = true
 
 Invalidate cached jacobians, etc.
 """
-invalidate_cache!(sys::AbstractSystem) = sys
+function invalidate_cache!(sys::AbstractSystem)
+    setmetadata(sys, MutableCacheKey, MutableCacheT())
+end
 
 function Setfield.get(obj::AbstractSystem, ::Setfield.PropertyLens{field}) where {field}
     getfield(obj, field)

src/systems/system.jl

@@ -8,6 +8,10 @@ end
 
 const MetadataT = Base.ImmutableDict{DataType, Any}
 
+abstract type MutableCacheKey end
+
+const MutableCacheT = Dict{DataType, Any}
+
 """
     $(TYPEDEF)
 
@@ -407,6 +411,7 @@ function System(eqs::Vector{Equation}, iv, dvs, ps, brownians = [];
         end
         metadata = meta
     end
+    metadata = Base.ImmutableDict(metadata, MutableCacheKey => MutableCacheT())
     System(Threads.atomic_add!(SYSTEM_COUNT, UInt(1)), eqs, noise_eqs, jumps, constraints,
         costs, consolidate, dvs, ps, brownians, iv, observed, Equation[],
         var_to_name, name, description, defaults, guesses, systems, initialization_eqs,