Refactor Compass to use heading

[philium]

This PR refactors the Compass component to use a heading to determine the orientation of the compass needle The viewpoint-based approach is still supported using a separate initializer.

For future consideration is whether autoHide should be a modifier. The way it is now, the default value lives in three places (in each of the initializers).

public extension Compass {
    func automaticallyHides(_ newAutomaticallyHides: Bool) -> some View {
        var copy = self
        copy.autoHide = newAutomaticallyHides
        return copy
    }
}

[philium]

This change facilitates storing the viewpoint components as separate state properties. For example, this sample could store the viewpoint rotation as a state property, allowing a binding to it to be passed directly to the slider. @clee088 @yo1995

[dfeinzimer]

I like the added flexibility these changes bring. I would also support an automaticallyHides modifier.

[philium]

@mhdostal I added some more doc. Please let me know what you think!

[mhdostal]

Looks great, thanks!