How to check if actor is in contact with a block/actor group?

Hi John! I'm resuming work on my platformer and am trying to figure out the best way of determining if a user is in contact with a block/actor in a group.

My use-case is I have my player who will walk off the edge of an obstacle. When he walks off the edge I want to get a notification that he's no longer in contact with the ground so that I can apply gravity.

See GIF for my current behavior:

I've accomplished this by adding the following methods to Actor:
```swift
public func isInContact(withBlockGroup group: Group) -> Bool {
for block in blocksInContact {
if block.group == group {
return true
}
}

return false
}

public func isInContact(withActorGroup group: Group) -> Bool {
for actor in actorsInContact {
if actor.group == group {
return true
}
}

return false
}
```

I have a plugin that watches for rectChanges and flips a flag which changes gravity behavior:
```swift
actor.events.rectChanged.addObserver(self) { (scene, player, stuff) in
if !player.isInContact(withBlockGroup: self.floorGroup) {
if self.isGrounded {
self.isGrounded = false
}
}
}
```

Is there a better/more efficient way of doing this? If not I'd be happy to open a PR with the change.