big refactoring of engine : unused methods completely removed; addPlugVerbose, add_plug, add_input, add_output removed; all code calling them modified (TODO : except in optional files circuits.py and engine_test.py); now, Plug.__init__() is responsible for adding the new Plug to its owner; Circuit.remove() had a logical error : if the component to be removed didn't belong to the circuit, it was modified by Circuit.remove() anyways; all methods organized by alphabetical order; externalization of strings