refactor code into modules

src/pg_rad/landscape/landscape.py

@@ -5,7 +5,7 @@ from matplotlib.patches import Circle
 import numpy as np
 
 from pg_rad.path import Path
-from pg_rad.sources import PointSource
+from pg_rad.objects import PointSource
 
 logger = logging.getLogger(__name__)
 

src/pg_rad/objects/sources.py

@@ -1,7 +1,7 @@
 import logging
 
 from pg_rad.objects import Object
-from pg_rad.isotope import Isotope
+from pg_rad.isotopes import Isotope
 
 logger = logging.getLogger(__name__)