plots: add option to reverse bins for a heatmap plot with an ordinal scale

[GraemeWatt]

For a heatmap plot with non-numerical bins, there could be an option to reverse the order of the bins on each axis. This option could appear as a tick box next to the selection of axis variable. For example, a correlation matrix could then have the diagonal components from top-left to bottom-right instead of from bottom-left to top-right. The former choice is common for correlation matrix plots from the ATLAS Higgs group (example). On the other hand, the latter choice is used elsewhere; see example from the CMS SUSY group with corresponding HEPData table.

Request from Tim Adye (ATLAS).