Fixed bug in spherical_geometry.f90
Fixed bug in spherical_geometry.f90

Bug used unstable expression in cart2{geog,sph} for phi; this
has been replaced with the more stable phi = atan2(y,x).

anowacki/seismo-fortran,anowacki/seismo-fortran,anowacki/seismo-fortran,anowacki/seismo-fortran