Native support for multiple agent types

Tracks agents in the model with a defaultdict.

This PR adds a new `agents` dictionary to the Mesa `Model` class, enabling native support for handling multiple agent types within models. This way all modules can know which agents and agents type are in the model at any given time, by calling `model.agents`.

NetLogo has had agent types, called [`breeds`](https://ccl.northwestern.edu/netlogo/docs/dict/breed.html), built-in from the start. It works perfectly in all NetLogo components, because it's a first class citizen and all components need to be designed to consider different breeds.

In Mesa, agent types are an afterthought at best. Almost nothing is currently designed with multiple agent types in mind. That has caused several issues and limitations over the years, including:

- #348
- #1142
- #1162

Especially in scheduling, space and datacollection, lack of a native, consistent construct for agent types severely limits the possibilities. With the discussion about patches and "empty" this discussion done again. You might want empty to refer to all agents or only a subset of types or single type. That's currently cumbersome to implement.

Basically, by always having dictionary available of which agents of which types are in the model, you can always trust on a consistent construct to iterate over agents and agent types.

- The `Model` class now uses a `defaultdict` to store agents, ensuring a set is automatically created for each new agent type.
- The `Agent` class has been updated to leverage this feature, simplifying the registration process when an agent is created.
- The `remove` method in the `Agent` class now uses `discard`, providing a safer way to remove agents from the model.

mesa/agent.py

@@ -20,19 +20,34 @@
 
 
 class Agent:
-    """Base class for a model agent."""
+    """
+    Base class for a model agent in Mesa.
+
+    Attributes:
+        unique_id (int): A unique identifier for this agent.
+        model (Model): A reference to the model instance.
+        self.pos: Position | None = None
+    """
 
     def __init__(self, unique_id: int, model: Model) -> None:
-        """Create a new agent.
+        """
+        Create a new agent.
 
         Args:
-            unique_id (int): A unique numeric identified for the agent
-            model: (Model): Instance of the model that contains the agent
+            unique_id (int): A unique identifier for this agent.
+            model (Model): The model instance in which the agent exists.
         """
         self.unique_id = unique_id
         self.model = model
         self.pos: Position | None = None
 
+        # Register the agent with the model using defaultdict
+        self.model.agents[type(self)].add(self)
+
+    def remove(self) -> None:
+        """Remove and delete the agent from the model."""
+        self.model.agents[type(self)].discard(self)
+
     def step(self) -> None:
         """A single step of the agent."""
 

mesa/model.py

@@ -8,6 +8,7 @@
 from __future__ import annotations
 
 import random
+from collections import defaultdict
 
 # mypy
 from typing import Any
@@ -41,6 +42,12 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
         self.running = True
         self.schedule = None
         self.current_id = 0
+        self.agents: defaultdict[type, set] = defaultdict(set)
+
+    @property
+    def agent_types(self) -> list:
+        """Return a list of different agent types."""
+        return list(self.agents.keys())
 
     def run_model(self) -> None:
         """Run the model until the end condition is reached. Overload as