refactor: store buffer index in ParameterIndex type

Author: AayushSabharwal

src/systems/index_cache.jl

@@ -11,13 +11,14 @@ end
 struct Nonnumeric <: SciMLStructures.AbstractPortion end
 const NONNUMERIC_PORTION = Nonnumeric()
 
-struct ParameterIndex{P, I}
+struct ParameterIndex{P, B, I}
     portion::P
     idx::I
     validate_size::Bool
 end
 
-ParameterIndex(portion, idx) = ParameterIndex(portion, idx, false)
+ParameterIndex(portion, idx, val = false) = ParameterIndex{typeof(portion), idx[1], typeof(idx)}(portion, idx, val)
+ParameterIndex(portion::Union{SciMLStructures.Tunable, SciMLStructures.Initials}, idx, val = false) = ParameterIndex{typeof(portion), idx, typeof(idx)}(portion, idx, val)
 ParameterIndex(p::ParameterIndex) = ParameterIndex(p.portion, p.idx, false)
 
 struct DiscreteIndex