This reworks the assistance check into a new sub class. There are also a number of functional changes:

• This more carefully resets the assistance state if not in a mode that checks assistance. Currently if you trigger alt assist and enter a VTOL mode and then re-enter FW at a later point while still under the set alt you will get no delay and no second warning message because the in_alt_assist and alt_error_start_ms are not cleared.
• Force flare no longer changes the assist state, instead assist checks for force flare. This means that the state is still where the user left it after the flare is done.
• All types of assist are checked at once. Currently we stop checking when the assistance should be triggered. This means in speed assist altitude is not checked. We have a delay for altitude assist and angle, so it is possible to come out of speed assist briefly before starting alt assist.
• This adds a clear delay of double the set trigger delay (angle and alt only). This prevents a momentary clearing of the threshold from stopping assistance. We currently have a trigger delay and no clear delay, so if oscillating around the threshold we favor not assisting this change means we favor assisting.

This will need a fair bit of testing, so far it has had none. I have opened the PR so we can get consensus on the desired behavior.

We might also want to re-work the messages, currently you can get a alt assisting message while still in speed assistance, we might want the messages to behave as before, so as soon as the speed is reached the alt assistance message is given. However it is quite nice to get the message early, it saves attempting the transition only to reach your set speed and find your too low.

We might also want to re-issue the angle and alt assistance warnings periodically.

A followup PR could move all these functions into a new file, quadplane.cpp it is getting big....