added set_llm method

Author: sanika-n

mesa_llm/llm_agent.py

@@ -3,4 +3,9 @@
 
 class LLMAgent:
     def __init__(self, api_key: str, model: str = "openai/gpt-4o"):
+        """Initialize the LLMAgent with a ModuleLLM instance."""
+        self.llm = ModuleLLM(api_key=api_key, model=model)
+
+    def set_llm(self, api_key: str, model: str = "openai/gpt-4o") -> None:
+        """Replace the current LLM with a new configuration."""
         self.llm = ModuleLLM(api_key=api_key, model=model)