ENH: refine geometry and field scaling for nastran and raw surfaces (#1600)

- enhancement and potential breaking change.

  Nastran surface writer previously used the keyword 'scale' for
  field scaling, which was applied uniformly to all output fields.

  Change the meaning of 'scale' to be geometric scaling
  (consistent with reading triSurfaceMesh etc).

  New 'fieldScale' entry is an optional dictionary of scaling
  parameters.

  Example,

      nastran
      {
          scale   1000;     // [m] -> [mm]
          fieldScale
          {
             "p.*"   0.01;  // [Pa] -> [mbar]
          }
      }