avoid h5py mode=None

xsocs/process/fit/peak_fit.py

@@ -153,7 +153,7 @@ class FitResult(object):
         FitTypes.GAUSSIAN: 'gauss_0',
         FitTypes.CENTROID: 'centroid'}
 
-    def to_fit_h5(self, fit_h5, mode=None):
+    def to_fit_h5(self, fit_h5, mode='a'):
         """Write fit results to an HDF5 file
 
         :param str fit_h5: Filename where to save fit results