Added type hints for return values

src/mdevaluate/__init__.py

@@ -25,7 +25,7 @@ def open(
     index_file: Optional[str] = None,
     charges: Optional[list[float]] = None,
     masses: Optional[list[float]] = None,
-):
+) -> coordinates.Coordinates:
     """
     Open a simulation from a directory.
 
@@ -101,7 +101,7 @@ def open(
     return coords
 
 
-def open_energy(file: str):
+def open_energy(file: str) -> pd.DataFrame:
     """Reads a gromacs energy file and output the data in a pandas DataFrame.
     Args:
         file: Filename of the energy file