new functions to deal with spherical geometry: lat-lon conversion, degree to radian and radian to degree conversion (as lambda functions),
radian angle function (in radians), alternative arc distance function (using haversine formula), and geo-interpolate (way finding of a point
on the great circle distance between two given points)
Merge pull request #532 from lanselin/newsphere

new functions to deal with spherical geometry: lat-lon conversion, degre...