Make heatmaps more interpretable

[Bergam0t]

• Add option to change counts to average per run (maybe this is the default?)
• And/or add option to display per run

At the moment it's a bit confusing because it's giving counts for all x runs

[amyheather]

It's in model exploration tab

e.g. viewing by clockstart, hover, see 1000+ people arriving at a given time, need to explain that its aggregating across runs, and maybe be able to view by run

[Bergam0t]

May also be nice to make these facetable in a similar way to the plot above it (though this could be raised as a separate issue)