initialization from x,y,z arrays

particle.py

@@ -49,6 +49,15 @@ class Particles:
                 attr[key] = pp[col[key],:,0].squeeze()
         return cls(num,time,attr,period)
 
+    @classmethod
+    def from_position(cls,x,y,z,time,period):
+        assert x.ndim==1 and y.ndim==1 and z.ndim==1,\
+                "'x','y','z' must be one dimensional arrays of length Np."
+        assert len(x)==len(y) and len(y)==len(z)
+        num = len(x)
+        attr = {'id': np.arange(1,num+1),'x': np.asarray(x),'y': np.asarray(y),'z': np.asarray(z)}
+        return cls(num,time,attr,period)
+
     def __getitem__(self,val):
         if isinstance(val,int):
             lo,hi = val,val+1