STYLE: align faceZone handling (functionObjects, sampling)

- pattern as per surfaceFieldValue::setFaceZoneFaces()

  1. define faceId, facePatchId assuming an internal face
  2. if actually a boundary face:
     - get facePatchId
     - ignore if emptyPolyPatch or coupledPolyPatch (neighbour side)
     - get patch relative faceId

  This currently seems to be the least amount of code clutter.

ENH: recover some memory my shrinking lists in fluxSummary

BUG: potentially trailing rubbish in the heatExchangerModel lists

- the final resize to length actually used was missing.
  Does not affect any released versions