Function physical_derivatives 

pub fn physical_derivatives<const NN: usize>(
    jacobian: &Jacobian,
    dn: &[[f64; 2]; NN],
) -> [[f64; 2]; NN]

dN_i/dx_c = sum_a J^{-1}[c][a] dN_i/dxi_a, one [dN/dx, dN/dy] per node.