Add sub-headings to group the mitigators

[jacgrout]

Sub-headings to separate out in the presentation beween

Accident & Emergency
Inpatients
Outpatients

as well as

Admission Avoidance
Efficiencies

[matt-dray]

Thanks Jac. Just to confirm, I think you mean to group up the charts like this (pointrange example)?

## Inpatients

### Admission avoidance

[Faceted plot of schemes/mitigators]

### Efficiencies

[Faceted plot of schemes/mitigators]

## Outpatients

etc

And presumably ignoring headers/subheaders if the user didn't select any mitigators under those headings.

Or did you mean to group up the mitigator selection options in the left-hand settings panel? (Being looked into as part of #43 and subsequently #98.)

[matt-dray]

On reflection, I think this issue was meant more in the context of the heatmaps, specifically (but I suspect it applies throughout the app).

There'd be some jiggery-pokery required, of course, as we'd be going from one to multiple plots. These sections would have to appear or be ignored conditionally on the user's inputs. I could imagine splitting the underlying dataset into a list of dataframes that's converted to a list of named plots that could then be spat out into the UI. NULL elements could be ignored and not plotted, nor have their sub-heading printed.

@craig-parylo and I chatted about some other (less complicated) solutions like adding an extra axis title that groups the mitigators. I thought {legendry} had a function for this, but I think it applies to continuous axes only.

[jacgrout]

Yes it was meant in the context of just the heatmaps.

[matt-dray]

This is something that would make particular sense in the context of #124.