faMesh does not have own registry

Functionality to add/problem to solve

faMesh does not have its own objectRegistry but uses the polyMesh one. This means e.g. that its data() conflicts with that of a fvMesh.

What does success look like, and how can we measure that?

Successfully run any finiteArea case with

DebugSwitches
{
    regIOobject 2;
}

(this debugswitch will trigger aborting upon double registration)