TUT: use general 'scale' instead of 'convertToMeters' in blockMeshDict

- although this has been supported for many years, the tutorials
  continued to use "convertToMeters" entry, which is specific to blockMesh.
  The "scale" is more consistent with other dictionaries.

ENH:
- ignore "scale 0;" (treat as no scaling) for blockMeshDict,
  consistent with use elsewhere.