ENH: support direct calculation of finiteArea edgeNormals (#2592)

- with geometryOrder=1, calculate the edge normals from the adjacent
  faces (area-weighted, inverse distance squared) and also
  use that for the Le() calculation.

  Includes the contributions from processor edge neighbours, so it
  should be consistent on both sides.

  This new method (consider as 'beta') contrasts with the current
  standard method that first calculates area-weighted point normals
  and uses the average of them for the edge normal.

  Enable for testing either with a controlDict OptimisationSwitch entry
  "fa:geometryOrder", or on the command-line:

      solverName -opt-switch=fa:geometryOrder=1