Add "arrivalCircle" to "nextPoint"

[panaaj]

Addition of arrivalCircle to nextPoint to allow an arrival alarm to be implemented.

[tkurki]

I think it would be useful to add this to https://github.com/SignalK/specification/blob/master/test/data/full-valid/course-full_tree.json#L48

Oh I just noticed that the hrefs in that doc put resources under a vessel...

[panaaj]

Added tests for nextPoint.arrivalCircle.
I have noticed two things in the test data that don't align with the text in the proposed CourseAPI.

1. href test value is under vessels ("href": "/vessels/vessels.urn:mrn:imo:mmsi:230099999/resources/waypoints/...")
   when in practice resources is at the root of ./signalk/v1/api.
2. arrivalCircleEntered notification is defined as notifications.arrivalCircleEntered where the proposed notification path in the Course API PR is notifications.navigation.course.arrivalCircleEntered

Should these be updated in the test file?

[tkurki]

I'll merge this, submit a new PR for fixes?

[tkurki]

Oh, please rebase. Would do it myself nicely if the branch were under this repo ;-)