Making plt-> dst conversion a bit clearer

new function plt.get_dst returns a dst object
function plt.save_dst forces you to provide a file name (and use get_dst)

ess/TraceWin.py

@@ -499,8 +499,10 @@ class plt:
         self.twiss_alpha = [[-self.sigma[j][i][i+1]/self.twiss_eps[j,i/2] for i in (0,2,4)] for j in range(len(self.Nelp))]
         self.twiss_alpha = numpy.array(self.twiss_alpha)
 
-
-    def save_dst(self, index,filename=''):
+    def get_dst(self, index):
+        '''
+        Returns the dst corresponding to the given index
+        '''
         import numpy
    
         dset=self[index]
@@ -516,8 +518,16 @@ class plt:
                            dset['yp'],
                            dset['phi'],
                            dset['E']]).transpose()
-        if filename:
-            _dst.save(filename)
+        return _dst
+
+    def save_dst(self, index,filename):
+        '''
+        Saves the dst at the specified index to file
+
+        Returns the same dst object.
+        '''
+        _dst=get_dst(index)
+        _dst.save(filename)
         return _dst