ENH: add absolute weighting for surfaceFieldValue (issue #567)

- can be useful either for flow-rate weighting where backflow
  is to be ignored in the average, or for flow-rate weighting
  on surfaces with inconsistent orientation.

  Reworked to code to make better use of Enum (the NamedEnum
  replacement). Enum doesn't require contiguous enumeration values,
  which lets us use bitmasking of similar operations to reduce
  duplicate code.