- the Le vector is calculated from (edgeVec ^ edgeNorm)
  and should be oriented in direction (faceCentre -> edgeCentre).

  If, however, the edgeNorm value is bad for any reason, the
  cross-product falls apart and Le vector is calculated as a zero
  vector!

  For these cases, revert to using (faceCentre -> edgeCentre)
  as a better approximation than a zero vector.

  In the future, will very likely switch calculating the edge normals
  directly from the attached faces, instead of from the attached
  points as is currently done, which should improve robustness.

ENH: expose fa:geometryOrder as a registered OptimisationSwitch

ENN: reuse polyMesh data (eg, faceCentres) if possible in faMesh

STYLE: add code lambdas and static functions to isolate logic