GitLab

- a valid() method (same as !empty() call) for consistency with other
  containers and data types

- a centre() method (same as midpoint() method) for consistency with
  other OpenFOAM geometric entities

- avoid duplicate code by relocating cellZone selection and bounding box
  sub-selection into cuttingPlane and cuttingSurfaceBaseSelection.
  Allows reuse by inherited classes (sampledPlane, surfMeshSamplePlane).