Allow the user to set arrival threshold

Now that high precision GNSS receivers are supported (as per #65), the ability for the user to set the arrival threshold is more important.
Consider this a refinement of #3. Some examples are:
- If a user has a receiver that as an accuracy of 1m, and they are searching for a manhole in an open field - an arrival threshold of 1m is a reasonable expectation.
- if a user in a city environement, using either an external receiver or internal position source and looking for a light pole (ie: something obvious), then an arrival threshold of 10-20m is reasonable.