BUG: Fix references to locator nodes
BUG: Fix references to locator nodes

Simple node attributes were used for storing node IDs of locator nodes. If the scene already contained nodes with the same model IDs then the locator nodes were hijacked.

Solution: use node references that take care of conflicting node IDs in the existing and imported scene.

IGSIO/OpenIGTLinkIO,IGSIO/OpenIGTLinkIO