use implicit conversions for lat,lon degs<->rads in grtcirc interface (#1321)

use implicit conversions for lat,lon degs<->rads in grtcirc interface (#1321)

* use implict conversions for lat,lon degs<->rads.

* use Position* return values.

* convert grtcirc i/f to PositionDeg, PositionRad.

* fix mac compile, denote converting ctor.

* inline PositionDeg/Rad converting ctors.

* respect PosotionX encapsulation.

* differentiate member names for PositionDeg, PositiionRad.  This can catch unintended usages where the type is not what was expected.

* clean up unneccesary usage of Waypoint::position.

* delete obsolete declaration.

* spell

* update garmin include

* consistenly unpack Positions to consts in grtcirc.

* provide default initializers for PositionX.