enable straight-line GPS distance measuring mode, for increased accuracy

[tshalif]

Straight-line mode in split-time should improve average speed, distance measuring accuracy, because the distance calculation is done between first, last location only, so it eliminates the GPS inaccuracy of all the points in between. This feature is enabled via a preference and should be turned-off unless the rowing course used for split-time measuring is relatively straight. Doubling-back in the middle of a straight line measurement will give negative distance..