ENH: more consistent min edge lengths (finite-area)

- for robustness with small edges (which can occur with snappy meshes),
  the Le() and magLe() are limited to SMALL (commit a0f1e98d).
  Now use factor sqrt(1/3) in the components to maintain magnitude of 1.

ENH: add fvMesh::unitSf() and faMesh::unitLe() methods

- simple wrappers around Sf()/magSf() and Le()/magLe() but with
  the potential for additional/alternative corrections.

STYLE: thisDb() in faMesh code to simplify future changes in storage

ENH: do not register finite-area geometric fields

- consistent with finite-volume treatment