ENH: trap negative blockMesh expansions and treat as their inverse

- A negative expansion ratio is geometrically invalid and will
  normally cause issues (FatalError).
  However, we can trap this type of input and interpret it as the
  inverse expansion ratio - ie, the expansion ratio in the opposite
  direction. This can be especially convenient when generating a blockMesh
  with a symmetrical expansion. It permits using the same expansion
  ratio (with a sign change) instead of providing the reciprocal values
  manually.

COMP: revert demand-driven point creation in blockMesh (db9b35b5)

- appears to have caused a dangling reference on some systems

ENH: add a lightweight ijkAddressing class

- it can be used for an i-j-k to linear lookup of meshes or fields,
  where applicable.