Oscilloscope cannot capture three channels

[bessman]

Oscilloscope.capture raises the follow exception when asked to capture three channels:

line 183, in _lookup_mininum_timegap
    return min_timegaps[channels_idx[channels]][self.trigger_enabled]

KeyError: 3

Fix the bug and add a test to check this behavior.

Thanks to @kartikaysharma01 for finding this bug.