Add assert statements to catch undefined components in system functio…

…n calls #95

payload_designer/components/masks.py

@@ -29,7 +29,8 @@ def __init__(self, mass=None, diameter=None, thickness=None, size: tuple = None)
 
     def get_area(self):
         """Get the clear aperture slit area."""
-
+        assert self.size is not None, "Size must be specified."
+
         area = self.size[0] * self.size[1]
 
         return area