refactor: change methods not using its bound instance to staticmethods

The method doesn't use its bound instance. Decorate this method with `@staticmethod` decorator, so that Python does not have to instantiate a bound method for every instance of this class thereby saving memory and computation. Read more about staticmethods [here](https://docs.python.org/3/library/functions.html#staticmethod).

src/raspa_ase/calculator.py

@@ -114,8 +114,8 @@ def execute(self, directory: Path | str, profile: RaspaProfile) -> None:
         """
         profile.run(directory, self.output_file)
 
+    @staticmethod
     def write_input(
-        self,
         directory: Path | str,
         atoms: Atoms,
         parameters: dict[str, Any],
@@ -148,7 +148,8 @@ def write_input(
         write_simulation_input(parameters, directory / SIMULATION_INPUT)
         write_frameworks(frameworks, directory)
 
-    def read_results(self, directory: Path | str) -> Results:
+    @staticmethod
+    def read_results(directory: Path | str) -> Results:
         """
         Read the results of a RASPA calculation.