Import totallength in ForwardDiff extension

[devmotion]

I think this is necessary and should fix downstream issues I just ran into:

  Got exception outside of a @test
  MethodError: no method matching totallength(::Vector{ForwardDiff.Dual{ForwardDiff.Tag{Pumas.Tag, ForwardDiff.Dual{ForwardDiff.Tag{Pumas.Tag, Float64}, Float64, 2}}, ForwardDiff.Dual{ForwardDiff.Tag{Pumas.Tag, Float64}, Float64, 2}, 1}})
  The function `totallength` exists, but no method is defined for this combination of argument types.
  
  Closest candidates are:
    totallength(::ForwardDiff.Dual)
     @ DiffEqBaseForwardDiffExt ~/.julia/packages/DiffEqBase/DnAEG/ext/DiffEqBaseForwardDiffExt.jl:504

[ChrisRackauckas]

fuck I bungled the hotfix sorry. I don't know how this wasn't caught in the first set of tests but I tested this locally on an autodiff solve case and we seem good now.