Allow the addReplace action to replace existing nodes while keeping the ...

...same node id.

This allows transparent swapping of a synth (e.g. updating synthdefs with a new version) while ensuring all references to the node remain valid. Allow this on Groups for consistancy, though it's functionally identical to freeing all of the Group's children.
please follow the coding style of the code of the file you are in. don't mix tabs and spaces. might sound picky, but helps the readability of the code!