improve consistency of 'normal' methods
In various places
normal() can be a unit normal or an area normal. In places where there can be ambiguity, propose to provide explicit
unitNormal() methods. In the next version (1812), phase out
normal() in favour of these and deprecate
normal(). In following version (1906), mark normal as
= delete or otherwise tag its usage to finalize changeover.
For the first step, I propose adding these new names, but not yet changing code to use them. This will get them into place for early adopters and also make it easier with bugfixes/backports between 1806 and the 1812-develop.