Support arrays beyond vectors?

[gdalle]

At the moment, gradient and jacobian fail for non-vector input.

MethodError: no method matching threaded_gradient!(::typeof(f_matrix_scalar), ::Matrix{Float64}, ::Matrix{Float64}, ::ForwardDiff.Chunk{4})
  
  Closest candidates are:
    threaded_gradient!(::F, ::AbstractVector, ::AbstractVector, ::ForwardDiff.Chunk{C}) where {F, C}
     @ PolyesterForwardDiff ~/.julia/packages/PolyesterForwardDiff/EsanS/src/PolyesterForwardDiff.jl:47