ENH: improve code consistency for {cell,face,point}Mapper (#3147)

- robuster and more reliable determination of inserted objects
  (previous code could produce false positives).

  Now also determine the number of inserted objects within the
  constructor (instead of simply storing a bool). This allows reuse
  in the address calculations to reduce overheads there.

BUG: dodgy short-circuit logic for insertedObjectLabels()

- as a quick short-circuit it previously created as demand-driven
  pointer with a zero elements. However, this meant that if this code
  was called first (before any other addressing), subsequent calls to
  the addressing would fail.

BUG: bad logic and lookup for faAreaMapper (#3147)

- was using labelHashSet but returning a bool and set using the
  addressing using the unfiltered objects!
  This is a latent bug since interpolated/inserted faces not
  currently supported anyhow.

ENH: use std::unique_ptr for demand-driven data