docs: add docstring for MassPointModel class

src/psimpy/simulator/mass_point_model.py

@@ -9,6 +9,8 @@ from beartype import beartype
 
 class MassPointModel:
 
+    """Simulate the movement of a masspoint on a topography."""
+    
     @staticmethod
     def _preprocess(elevation, x0, y0):
         """