odd/inconsistent handling of processor patches

Various items, in no particular order:

• processorFaPatchField updateInterfaceMatrix uses add to apply += whereas the fv version calls addToInternal with the add value flipped. Looks like a long standing bug, which was inconsistently handled by scalar/non-scalar versions (see commit 0de1df73).

• processor patchFields have hit/miss approach to transformCoupleField. In some cases the scalar versions are protected with an is_arithmetic, but not consistently.

@andy @Mattijs @kuti