This command is not documented in the User Guide as it is intended only for test
purposes. Out of bounds values are trapped.

It sets up a SubSynth engine in the designated part number in the range 1 to 16,
ensuring that the part is enabled. If no part number is given it assumes part 1.

It calls the same routine that the banks code uses to set up an instrument when it
creates a new bank root, but returns as if a normal part change had been made.
