improve lagrange point generator

should maybe do an explicit enumeration instead of filtering

src/operator.jl

@@ -127,8 +127,8 @@ function quadrature_composite_lagrange_midpoint(p)
     points = Matrix{Float64}(undef, 2, n)
 
     k = 0
-    for I in Iterators.product(ntuple(_ -> 0:p, d_ + 1)...)
-        I[1] + I[2] + I[3] != p && continue
+    for I in Iterators.product(ntuple(_ -> 0:p, d_)...)
+        sum(Tuple(I)) > p && continue
         k += 1
         weights[k] = 1 / n
         points[1, k] = I[1] / p