Added modulo function

propagate.c

@@ -172,6 +172,15 @@ orbit_t classel(int ep_year,double ep_day,xyz_t r,xyz_t v)
   return orb;
 }
 
+// Return x modulo y [0,y)
+double modulo(double x,double y)
+{
+  x=fmod(x,y);
+  if (x<0.0) x+=y;
+
+  return x;
+}
+
 orbit_t rv2el(int satno,double mjd,xyz_t r0,xyz_t v0)
 {
   int i,imode;