Add integer locators and formatters to `SliceWidget`

[dstansby]

For small data ranges, the x-axis of the slice widget shows floating point numbers:

Since the data only ever falls on integer values, it would be good to set the ticks to be at integer locations, and the tick values to be formatted as integers. See https://matplotlib.org/stable/api/ticker_api.html for more info on how to do this.