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.