Add shifting to etr

[dougollerenshaw]

Adds an option to event_triggered_response to shift responses instead of interpolating Adds an example notebook to the examples folder showing the syntax and results for event_triggered_response` using both the interpolate=True and interpolate=False methods Adds two functions to general_utilities:

• index_of_nearest_value
• slice_inds_and_offsets

Both have associated tests