creating infout.regressor_names list

corrected and error in _concatenate_info - when regressor and regressor_names are not defined, the fix fails. Now the regressor_names are created if they were not defined by the user.

nipype/algorithms/modelgen.py

@@ -562,6 +562,7 @@ def _concatenate_info(self, infolist):
             # insert session regressors
             if not hasattr(infoout, 'regressors') or not infoout.regressors:
                 infoout.regressors = []
+                infoout.regressor_names = []
             onelist = np.zeros((1, sum(nscans)))
             onelist[0, sum(nscans[0:i]):sum(nscans[0:(i + 1)])] = 1
             infoout.regressors.insert(