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.
Although: https://github.com/tiimgreen/github-cheat-sheet#ignore-whitespace