UTCI heatmap plots: Redesign clearer legend

[jamesramsden-bh]

I've had comments on more than one occasion that the UTCI heatmaps need a legend, despite the fact that the 'show_colorbar' is effectively a legend and is enabled by default.

I think that the current legend just isn't clear enough, particularly in its location along the bottom. I propose moving the legend to the side and adding some heat stress info.

Current plot:

Proposed plot:

cc @tg359