FEM: inout tools, add some comment in the regard of NaN

src/Mod/Fem/feminout/importToolsFem.py

@@ -365,7 +365,7 @@ def calculate_principal_stress(i):
         return (eigvals[0], eigvals[1], eigvals[2], maxshear)
     except:
         return (float('NaN'), float('NaN'), float('NaN'), float('NaN'))
-
+    # TODO might be possible without a try except for NaN, https://forum.freecadweb.org/viewtopic.php?f=22&t=33911&start=10#p284229
 
 def get_span(node_items):
     positions = []  # list of node vectors