Please add parameter 'end_times' for tick.hawkes.HawkesExpKern.fit()

[yaowang74]

In tick/tick/hawkes/model/base/model_hawkes.py, lines 91 to 94, we have

91 end_times = self._end_times 92 if end_times is None: 93 non_empty_events = [[r for r in e if len(r) > 0] for e in events] 94 end_times = np.array([max(map(max, e)) for e in non_empty_events])

if non_empty_events list is also empty, line 94 will spit an error on using max function on empty list.

In practice, it would be great if we can have an option to manually specify the 'end_times' for .fit() method. Thank you.