blockMesh:

  renamed genBlockMesh.C -> blockMeshApp.C for the -doc option

  Followed Eugene's suggestions and moved blockMeshDict out of polyMesh/.

  Rationale:
  blockMeshDict is not a polyMesh or part of a polyMesh, thus it doesn't
  really belong in the polyMesh/ dir anyhow. Moving it to constant/ or
  constant/<region>/ improves the overview and eases cleanup of polyMesh/ as
  well. For compatibility, constant/polyMesh/ or constant/<region>/polyMesh/
  will be searched if the new locations fail.