pub fn shape_function_ty() -> Expr
ShapeFunction : ReferenceTriangle → Nat → (Real → Real → Real)
The k-th Lagrange P1 shape function on the reference triangle: φ₁(ξ,η) = 1 - ξ - η, φ₂(ξ,η) = ξ, φ₃(ξ,η) = η.