Use CLLocation.course for heading over CLHeading

[jixer]

Fixes #19 .

Changes Proposed in this pull request:

• iOS: Derive Position.Heading value from CLLocation.course (GPS heading) instead of CLHeading (device heading)
• iOS: Remove logic that receive compass heading
  • Remove CLLocationManager.startUpdatingHeading call
  • Remove locationManager(_:didUpdateHeading:) delegate

Test results: Several tests were ran with the updated library using iOS 12 on iPhone 6S Plus. Tested the new logic while app was in the foreground in landscape and portrait modes. Also tested logic while app was in the background with the screen off and the device in landscape and portrait modes. The map results can be viewed here: https://drive.google.com/open?id=1YdmfvLhoYwKhJmtX1L9V0UUTGL2ljUlO&usp=sharing

Future considerations:

• Add back CLHeading value, but introduce a new object or value inside Position (e.g., CompassHeading) to store the result

[jixer]

Hi all, sorry to be a pest. I know that Xamarin.Essentials is more actively maintained than this repo, but we're not ready to fully migrate until the Location Update PR is finished. Is someone able to look into this. I've been using a locally built NuGet from this PR at my client, but I'm hoping to point back to the main NuGet repo.

[rohitvipin]

Could you resolve your conflicts?