Fix parsing of thickness field in stackup

Related #58

kicad.py

@@ -647,8 +647,14 @@ def _initStackUp(self):
                             self.copper_thickness if layer_type<=32 else self.layer_thickness)
                     if layer_type <= 31:
                         last_copper = offset
+                    # Some layer may have more than one thickness field.
                     if isinstance(t, SexpList):
-                        t = t[0][0]
+                        t = t[0]
+                    # And for some thickness field, there may be additional
+                    # attribute, like (thickness, 0.05, locked).
+                    if not isinstance(t, (float, int)):
+                        t = t[0]
+
                     offset -= t
                     self.stackup.append([unquote(layer[0]), offset, t])
                 # adjust offset to make the last copper's upper face at z = 0.