UI changes to website enabling case study as a sub category

[jwarner8]

Currently the left hand panel is populated through the category argument of recipes. If we plots lots of spatial plots, for example, the Surface Spatial Plot will contain ~40 plots. If we run for multiple case studies, this number will scale (so 14 case studies - a two week trial) - we will have 14*40 plots, which will become unmanageable to navigate quite quickly.

I wonder if we could create an index file that populates the menu with subcategories, so that you would have something like this;

Surface Spatial Plot
20191014T0000Z
20191015T0000Z
20191016T0000Z
Aggregated
Histogram
20191014T0000Z
20191015T0000Z
20191016T0000Z
Aggregated

We could pass the case study (cycle point) through to the recipe via the includes or otherwise, and use this to populate a menu? E.g. in the recipe;

category: Surface Spatial Plot Difference
category2: $CYCLEPOINT
title: Surface $VARNAME difference ($BASE_MODEL - $OTHER_MODEL)
description: Extracts and plot the difference in surface $VARNAME for all times.