IsolationSelection: Remove isAssociatedToEG decoration.

IsolationSelection: Remove isAssociatedToEG decoration.

IsolationCloseByCorrection was using the decoration isAssociatedToEG to tell
if a cluster was matched to an EM object.  This decoration was left unlocked,
resulting in warnings further on.  But that decoration is used only within
this tool, so this information would be better maintained locally.
Add it to the internal cache used by the tool rather than having it
as a decoration.