ENH: Adding subMesh option to momentumError and div FOs

• Adding subMesh capabilities to momentumError and div FOs.
  • A subMesh is created from cellZones to compute FOs.
  • The operators (div, etc) are only calculated in the subMesh.
  • Zero field is written in the non-subMesh cells.
• Optionally, halo cells can be added to the cellZones.
• New helper class to handle the subMesh creation and field mapping.
• TUT: add an example to tutorials/compressible/rhoSimpleFoam/aerofoilNACA0012