Redundant subdimensions being created when finalizing subdomains

[rhodrin]

When attaching M SubDomain's to an N-dimensional grid M*N SubDimension's are currently being created, but often some SubDimension's can, and should, be re-used.

Therefore, during the finalization of a SubDomain a check should be carried out to see if a previously created SubDimension can be used. An easy way to this would be to at, e.g., a grid._subdimensions property which is checked against and appended too during the finalization of each SubDomain.