Implemented ganging for CompoundNumericPlugs.

This is provided at the API level by new methods on the CompoundNumericPlug. It is then made available to the user on the popup menu on plug fields, and via a Ctrl-G shortcut when the cursor is in
a plug field.

Fixes #402.
Merge pull request #405 from johnhaddon/ganging

Ganging for CompoundNumericPlugs.