added method to reverse beam direction.

pykat/utilities/optics/gaussian_beams.py

@@ -146,6 +146,10 @@ class gauss_param(object):
     @imag.setter
     def imag(self, value): self.__q.imag = SIfloat(value)
 
+    # reverse beam direction 
+    def reverse(self):
+        self.__q = -1.0 * self.__q.real + 1j * self.__q.imag
+    
 class HG_gauss_beam(object):
     
     def __init__(self, qx, qy=None, n=0, m=0):