calculate distance from station and event coordinates
>>> t = SacIO()
>>> t.SetHvalue('evla',48.15)
>>> t.SetHvalue('evlo',11.58333)
>>> t.SetHvalue('stla',-41.2869)
>>> t.SetHvalue('stlo',174.7746)
>>> t._get_dist()
>>> print(round(t.GetHvalue('dist'), 2))
18486.53
>>> print(round(t.GetHvalue('az'), 5))
65.65415
>>> print(round(t.GetHvalue('baz'), 4))
305.9755
The original SAC-program calculates the distance assuming a average radius of 6371 km. Therefore, our routine should be more accurate.