++dmm_loop <0|1> to disable the HP3478A DMM loop, if for some reason the
DMM app loop is interfering with the GPIB bus when used as a USB-GPIB
controller
Add diode mode, math (min/max/avg), temp - k type thermocouple and
thermistor (only 5k tho..), restore mode to the one we came from when
exiting feature.
Add all features to USB-CDC interface, clean up state a bit (union)