Expand Q-Q plot functionality

Expand the functionality of the Q-Q plots to allow plotting over a single time, multiple ensemble members, etc.

'List of coordinates to collapse the percentiles over'. Assume if a user wants to do separate QQ plots for each lead-time, they would have to re-run the suite? If I had a cube of dim (time, ensemble, lat, lon), and I wanted to compute QQ plots for each time, would I supply ensemble, lat, lon coordinates to this COORDINATE_LIST and it would produce QQ plots for each time? Might be worth including an example above in extended help text.

_Originally posted by @jwarner8 in https://github.com/MetOffice/CSET/pull/642#discussion_r1694886458_

Start by locating the Q-Q plot implementation and the COORDINATE_LIST option described in the issue. Clarify how collapsing ensemble, latitude, longitude, and time coordinates should determine separate plots, then check the extended help text for an example. Done means the requested single-time and multiple-ensemble-member cases are supported and documented.

Written by the indexing model from the issue text.