WIP: Landing switch

[SimonWilks]

Notes for consideration:

• fault detection: detecting stuck a stuck switch state requires a check right after launch not as it happens otherwise we keep getting false negatives once the airframe starts shaking/moving. When checking for motion we should use larger limits than what is used for a non-switch takeoff, perhaps updating the parameters is enough.
• multiple switches on landing/takeoff: on landing we want all or the majority of switches to be on. for takeoff we want all or the majority off. perhaps we should be comparing the number of switches on for launch by checking how many are on after arming (will catch the situation where one or more are hanging in the air on non-level ground).
• use resistors to measure the voltage and so workout how many switches are on.