Option for manual control of multi-speed supercharger

Currently the most up-to-date system in place (using the new_supercharged
config setting) allows for multi-stage supercharger support, but it only
allows for automatic gear shifting based on the parameters that are
configured. This is a good start, however there are many, many classic
aircraft with superchargers where the shifting was not automatic, but pilot
controlled - the pilot had full control over what gear the supercharged is
placed in, with a shifter lever in the cockpit. I would propose that the
new_supercharged entry gain another enum value: 3 - Use for manual multi
speed supercharger support. I would then further propose that the simvar
RECIP ENG SUPERCHARGER ACTIVE GEAR:index be made writable, and that events
be created for supercharger gear up, supercharger gear down, and supercharger
gear set.