Compute saturation intervals ala IBL

[oliche]

This PR ports the saturation method from IBL. It will introduce a new data type for holding information on events / periods of noise / saturation etc. This will take the form of a numpy recarray with fields (draft names):

[start_sample_index, stop_sample_index, segment_index, channel_x_start, channel_x_stop, channel_y_start, channel_y_stop, method_id]

This will also require:

• update 'silence_artefacts.py` to output this new data type
• update silence_periods.py to handle this new data type

Other notes:

• remove_artefacts.py handles a slightly different use case (triggers at known timepoints contaminating the recording, which may also be imputed with a model-based approach) and so is not considered here

TODO:

• document the saturation values for different probes. Note that gain can be set on the amplifier so the saturation value is not entirely dependent on the probe type.
• the default velocity could be some ratio of the saturation value
• We can do some confidence checks on the input to ensure it's in microvolts
• for the two main arguments: add pre-sets and document? or at least reccomend values in documentation probably easier

[alejoe91]

Can we apply this directly in the int16 space?