Fix DiffCacheAllocatorAPIWrapper conversion errors

[SebastianM-C]

Fixes

ERROR: MethodError: Cannot `convert` an object of type ModelingToolkitBase.DiffCacheAllocatorAPIWrapper{Float64} to an object of type PreallocationTools.DiffCache{Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(costfn), Float64}, Float64, 2}}, Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(costfn), Float64}, Float64, 2}}}
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  (::Type{PreallocationTools.DiffCache{T, S}} where {T<:AbstractArray, S<:AbstractArray})(::Any, ::Any, ::Any, ::Any)
   @ PreallocationTools ~/.julia/packages/PreallocationTools/K2Foo/src/PreallocationTools.jl:82
  convert(::Type{T}, ::T) where T
   @ Base Base_compiler.jl:133

Stacktrace:
  [1] ModelingToolkitBase.DiffCacheAllocatorAPIWrapper{…}(cache::ModelingToolkitBase.DiffCacheAllocatorAPIWrapper{…})
    @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/jyweh/src/systems/diffeqs/basic_transformations.jl:1164
  [2] _broadcast_getindex_evalf
    @ ./broadcast.jl:699 [inlined]
  [3] _broadcast_getindex
    @ ./broadcast.jl:672 [inlined]
  [4] _getindex
    @ ./broadcast.jl:620 [inlined]
  [5] getindex
    @ ./broadcast.jl:616 [inlined]
  [6] copy
    @ ./broadcast.jl:933 [inlined]
  [7] materialize
    @ ./broadcast.jl:894 [inlined]
  [8] macro expansion
    @ ~/.julia/packages/Setfield/ZezIj/src/sugar.jl:198 [inlined]
  [9] (::ModelingToolkitBase.var"#_getter#742"{…})(valp::NonlinearProblem{…}, initprob::ODEProblem{…})
    @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/jyweh/src/systems/problem_utils.jl:851
 [10] (::ModelingToolkitBase.var"#initprobpmap_split#746"{…})(prob::ODEProblem{…}, initsol::NonlinearProblem{…})
    @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/jyweh/src/systems/problem_utils.jl:974
 [11] get_initial_values(prob::ODEProblem{…}, valp::ODEProblem{…}, f::ODEFunction{…}, alg::SciMLBase.OverrideInit{…}, iip::Val{…}; nlsolve_alg::Nothing, abstol::Nothing, reltol::Nothing, kwargs::@Kwargs{})
    @ SciMLBase ~/.julia/packages/SciMLBase/Ag5zT/src/initialization.jl:325
 [12] get_initial_values
    @ ~/.julia/packages/SciMLBase/Ag5zT/src/initialization.jl:267 [inlined]
 [13] maybe_eager_initialize_problem(prob::ODEProblem{…}, initialization_data::SciMLBase.OverrideInitData{…}, lazy_initialization::Nothing)
    @ SciMLBase ~/.julia/packages/SciMLBase/Ag5zT/src/remake.jl:1576
 [14] remake(prob::ODEProblem{…}; f::Missing, u0::Missing, tspan::Missing, p::MTKParameters{…}, kwargs::Missing, interpret_symbolicmap::Bool, build_initializeprob::Type, use_defaults::Bool, lazy_initialization::Nothing, _kwargs::@Kwargs{})
    @ SciMLBase ~/.julia/packages/SciMLBase/Ag5zT/src/remake.jl:392
 [15] costfn(theta::Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(costfn), Float64}, Float64, 2}})
    @ Main ./REPL[19]:3

by adding an explicit constructor that does the conversion

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.