Allow image markers to plot agents

[Holzhauer]

Summary

Enable images as markers for plotting agents and apply the feature to the Schelling model example

Motive

In may cases images are a preferable means to distinguish agents in the visualisation.

Implementation

In _scatter() it is checked whether the value for marker contains a valid file resource. If so, an image is created and zoomed (_get_zoom_factor) to the size of a grid cell (it is required to postpone calls of _scatter() in draw* methods because x and y limits need to be set earlier). An AnnotationBbox is used to display the image on the grid.

Usage Examples

The Schelling model was updated to apply image markers (and AgentPortrayalStyle) depending on agent type and state:

(Agents' step method was changed and assign_state introduced. Now the initial number of happy agents is shown correctly in the figure.)

Additional Notes

It seems that figure size is adjusted in solara after its initilisation. That's why CORRECTION_FACTOR_MARKER_ZOOM = 0.6 was introduced. There might be a better approach...

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔴 +4.7% [+4.0%, +5.3%]	🔵 -0.2% [-0.4%, +0.0%]
BoltzmannWealth	large	🔵 -0.5% [-1.2%, +0.1%]	🔵 +0.1% [-2.8%, +3.0%]
Schelling	small	🔴 +40.6% [+40.3%, +40.8%]	🔵 -2.2% [-2.4%, -1.9%]
Schelling	large	🔴 +119.6% [+117.7%, +122.0%]	🟢 -13.5% [-14.6%, -12.6%]
WolfSheep	small	🔵 +0.6% [+0.2%, +1.1%]	🔵 -0.1% [-0.3%, +0.2%]
WolfSheep	large	🔵 +0.2% [-0.4%, +0.9%]	🔵 -0.0% [-1.1%, +1.0%]
BoidFlockers	small	🔵 -0.3% [-0.9%, +0.4%]	🔵 -1.4% [-1.7%, -1.2%]
BoidFlockers	large	🔵 -1.8% [-2.1%, -1.4%]	🔵 -2.3% [-2.6%, -1.9%]