STYLE: add compile-time deprecated message for face/triangle normal()

- in 1812 propagated through the distinction between areaNormal and
  unitNormal (issue #885).

  In older versions, the normal() always meant the area-normal for
  certain of these primitive.

  However, the .org version changed this to now return the unit-normal
  instead, but with the same method name. Thus add the deprecated
  message to avoid future inadvertent uses of normal() without being
  certain which one is being meant.