Add a fixed heater option in the menu

[PBrunot]

New option "MODEL" with values:

• AUTO : let Unisolder determine the heater based on resistor value
• Model name : force detection of the chosen heater

Option is persisted in EEPROM.

I've tested this change with a JBC 245 succesfully. Unisolder detects open heater in case the instrument is disconnected.

The goal is to eliminate spurious issues with wrong model / no instrument detected in environments where only one heater is available.