- weight fields are combined by multiplication

  - volFieldValue:
    * 0-N scalar fields

  - surfaceFieldValue:
    * 0-N scalar fields
    * 0-1 vector fields

  In some cases this can be used to avoid creating additional
  fields.

      weightFields  (rho U);
  vs.
      derivedFields (rhoU);
      weightField   rhoU;