add agents property to all spaces

[quaquel]

This adds an agents property to all spaces which returns an AgentSet containing the agents in the space. See #2416 for background discussion

TODO

• tests

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -1.3% [-3.8%, +1.3%]	🔵 -0.1% [-0.2%, +0.0%]
BoltzmannWealth	large	🔵 +0.1% [-0.8%, +1.0%]	🔵 +1.5% [-0.3%, +3.1%]
Schelling	small	🔵 -0.2% [-0.7%, +0.3%]	🔵 +0.2% [-0.1%, +0.5%]
Schelling	large	🔵 +0.0% [-1.8%, +2.1%]	🔵 +3.1% [-2.2%, +8.2%]
WolfSheep	small	🔵 +0.0% [-0.7%, +0.6%]	🔵 +0.9% [+0.5%, +1.2%]
WolfSheep	large	🔵 -1.0% [-2.2%, +0.3%]	🔵 +0.0% [-4.3%, +4.1%]
BoidFlockers	small	🔵 +1.6% [+0.5%, +2.8%]	🔵 +1.0% [+0.2%, +1.7%]
BoidFlockers	large	🔵 +1.9% [+1.3%, +2.5%]	🔵 +0.9% [+0.3%, +1.6%]

[EwoutH]

Looks good, thanks!