note Rayleigh anomaly test case

DiffractionGratings/grating3D_data_hole.geo

@@ -2,6 +2,7 @@
 // Ndofs 1116912
 // solve wall time mumps  : 634.318s - R00=0.2432916553175037
 // solve wall time pastix : 966.046s - R00=0.2432916553175064
+// beware : very near a Rayleigh anomaly
 
 nm  = 1000;
 pp1 = "1Incident Plane Wave";