warp Reference Point to current GPS position

[JohnDenker]

This is related to #702.

It would be nice to have a button (perhaps on the Edit popup) that would warp the Accuracy page Reference Point to the current GPS location.

Rationale: Use case: Suppose are at a nice benchmark. It would be nice to drop the Accuracy page's Reference Point there. Then sit there and see how well the GPS holds that location. Or go for a jaunt and then see how well the GPS reproduces that location when you return.

Also..... Any modification to the Reference Point should log a NMEA "WPL" sentence, so that users can reconstruct the Accuracy measurement later.

Along the same lines: There should be a WPL at the top of every new log file, to record the Reference Point currently in effect.