[ENH] - Add spike time implementation of circular shuffle

[TomDonoghue]

From @LucaKolibius (but he didn't want to open an issue....)

Suggestion: have an option / implementation for circular shuffle that operates on spike times (adding different offsets) without converting to spike trains.

[LucaKolibius]

(How I implemented it):
startRec = 0
endRec = np.max(task.session['end_time']) # Or however you store recording end
random_int = np.random.randint(startRec, endRec + 1) ## add a random time (this should be floats, but I doubt it makes a difference
spikes_shuf = spikes + random_int # spikes in timestamps
spikes_shuf[spikes_shuf > endRec] -= endRec