Add get_start_time, get_end_time, and get_last_spike_frame to BaseSorting

[h-mayorquin]

BaseSorting has no public way to query the start or end time of a segment. BaseRecording has both (#3623). This PR adds get_start_time and get_end_time to BaseSorting, plus a get_last_spike_frame helper. With a registered recording, get_end_time returns the recording's end time. Without one, it returns the time of the last spike in the segment (the best the sorting can do on its own). get_start_time reads _t_start directly in both cases.

Adding these accessors however creates an ambiguity: some sorting extractors set _t_start on their segments at init. For example, neo sorting extractors set _t_start from the signal stream (e.g. Neuralynx infers it here). If that sorting registers a recording that also starts at 100.0s, get_start_time would need to decide which value to return.

The decision here is that registering a recording is a statement that "these are the right timestamps" (see #4519). In consequence, register_recording now copies the recording's start times into the sorting segments' _t_start. This replaces whatever the extractor had set at init, so get_start_time returns consistent values.