move _setup_agent_registration into Model.__init__

[quaquel]

For people transitioning from mesa 2 to mesa 3, we still had some nice warnings if in model super was not called. For 3.1, this won't be needed anymore, and it simplifies the code. This PR moves the setup of agent registration data structures into Model.__init__ and simplifies register_agent.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🟢 -9.0% [-10.1%, -7.8%]	🔵 -1.0% [-1.2%, -0.8%]
BoltzmannWealth	large	🔵 -3.2% [-5.6%, -1.6%]	🔵 -3.0% [-4.3%, -1.9%]
Schelling	small	🔵 -0.7% [-1.1%, -0.4%]	🔵 -1.5% [-1.6%, -1.4%]
Schelling	large	🔵 -0.7% [-0.9%, -0.4%]	🔵 +1.2% [-0.3%, +2.2%]
WolfSheep	small	🔵 -1.1% [-1.5%, -0.7%]	🔵 -0.8% [-1.0%, -0.7%]
WolfSheep	large	🔵 -1.7% [-3.5%, -0.6%]	🔵 -1.8% [-3.4%, -0.3%]
BoidFlockers	small	🔵 -2.8% [-3.4%, -2.3%]	🔵 -1.9% [-2.6%, -1.0%]
BoidFlockers	large	🔵 -3.1% [-3.6%, -2.4%]	🔵 -1.8% [-2.4%, -1.1%]

[EwoutH]

CI errors are unrelated, so go ahead. Thanks!

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -0.7% [-2.1%, +0.8%]	🔵 -0.3% [-0.5%, -0.1%]
BoltzmannWealth	large	🔵 -1.7% [-2.5%, -0.7%]	🔵 -1.2% [-3.3%, +0.8%]
Schelling	small	🔵 -2.6% [-2.9%, -2.4%]	🔵 -0.5% [-0.7%, -0.3%]
Schelling	large	🔵 -1.8% [-2.2%, -1.3%]	🔵 +1.4% [-0.1%, +3.1%]
WolfSheep	small	🔵 -0.6% [-1.1%, -0.2%]	🔵 +0.2% [-0.2%, +0.6%]
WolfSheep	large	🔵 +2.1% [+1.4%, +2.7%]	🔴 +8.4% [+5.7%, +10.4%]
BoidFlockers	small	🔵 -2.6% [-3.1%, -2.0%]	🔵 -0.1% [-1.0%, +0.8%]
BoidFlockers	large	🔵 -2.0% [-2.6%, -1.2%]	🔵 +1.2% [+0.8%, +1.7%]