Minor rounding error fix for the latitude and longitude strings.

Minor rounding error fix for the latitude and longitude strings.

This is for the FGProperties::getLongitudeString() and
FGProperties::getLatitudeString() functions.  The previous algorithm was to
round the degrees up by the smallest fraction required to prevent a round up to
60 minutes or seconds, and then round down the final minutes or seconds by the
same fraction.  The new algorithm is to detect if the final minute or seconds
will be rounded to 60 by the string formatting and, if so, the higher unit
(degrees or minutes) is incremented by one, and the lower unit decremented by
60.