Raiding units aren't flagged inactive, update code and docs

[Bumber64]

Add Units::isInPlay fn to check isActive then scan units.active. Useful when you've got a unit pointer, but you don't know if it's in the active units vector. (DF unsets the inactive flag a few ticks after raiding units leave the map.)

Updated some code and docs using this info. I would've changed the isActive function, but there may be performance concerns. Would be great if Bay12 could just use a flag for this, but we can't count on raiding bugs being fixed this decade.

Made getUnitsInBox rule out inactive units by default, since death and map edge locations aren't where the unit actually is.

[Bumber64]

Couldn't find any places in existing code that would benefit from a Units::isInPlay. linear_index can be used if someone needs to scan world.units.all.

[Bumber64]

hf.info.whereabouts.army_id was mentioned as a way to get the army so we could check for raiding. If that's only set when they're off the map, that could work.

hf.info.whereabouts.site_id can be checked to rule out exiled units after arrival at their new site, but it gives the lair for megabeasts currently on the map.

[Bumber64]

Replaced by #4959