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 areaNormal() or 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.

@Nilsson @johan_roenby