Speed up evaluation of instance uri expression + avoid creating seperate instanceNodes for identical models with different scale.

The construction of the instanceUri  took a long time, so I've added a map, for faster lookup of already created URI's

Currently models are counted as unique if their scaling differs, causing alot of clones to be created.
I'm using a default scaling of 1 for models when generating the lookup key, to avoid this.
This doesn't seem tobreak anything on my end, but  maybe you have some insight I don't ? :)

kind regards.

Jeroen