Making sure behavior is consistent wrt Py2/Py3

Range returns an iterable in Python3. Making sure result is a list
always.

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>

lifelines/estimation.py

@@ -1194,10 +1194,10 @@ def _compute_baseline_hazard(self):
 #### Utils ####
 def get_index(X):
     if isinstance(X, pd.DataFrame):
-       index = list(X.index)
+        index = list(X.index)
     else:
-       # If it's not a dataframe, order is up to user
-       index = range(X.shape[0])
+        # If it's not a dataframe, order is up to user
+        index = list(range(X.shape[0]))
     return index