add norm gradient calculation

src/mesh.jl

@@ -400,6 +400,23 @@ end
 
 mesh_size(mesh) = mapreduce(cell -> diam(mesh, cell), max, cells(mesh))
 
+# https://en.wikipedia.org/wiki/Incircle_and_excircles_of_a_triangle#Radius
+function inradius(mesh, cell)
+    A = SArray{Tuple{ndims_space(mesh), nvertices_cell(mesh)}}(
+	view(mesh.vertices, :, view(mesh.cells, :, cell)))
+    a, b, c =
+        norm(A[:, 1] - A[:, 2]),
+	norm(A[:, 2] - A[:, 3]),
+	norm(A[:, 3] - A[:, 1])
+    s = (a + b + c) / 2
+    return sqrt((s - a) * (s - b) * (s - c) / s)
+end
+
+function norm_gradient(mesh, cell)
+    min_inradius = mapreduce(cell -> inradius(mesh, cell), min, cells(mesh))
+    return 2 * sqrt(6) / (2 * min_inradius)
+end
+
 function elmap(mesh, cell)
     @boundscheck cell in axes(mesh.cells, 2)
     # TODO: can be improved, how?