Alex Romosan:

Alex Romosan:

* Use "const string&" rather than "string" in function calls when appropriate.
* Use "const Point3D&" instead of "Pint3D" in function calls when appropriate.
* Improved course calculation in calc_gc_course_dist()
* Safer thread handling code.

Vassilii Khachaturov:

Dont use "const Point3D&" for return types unless you're absolutely sure.

Erik Hofman:

* Use SGD_(2)PI(_[24]) as defined in simgear/constants.h rather than
  calculating it by hand every time.