refactor: organize src modules by category

tests/test_integration.py

@@ -5,9 +5,9 @@ are the load-bearing tests for the "flux doubling" coupling mechanism.
 """
 import numpy as np
 import pytest
-from tank import Tank
-from pipe import Pipe
-from solver import run
+from tank_pipe.tank import Tank
+from tank_pipe.pipe import Pipe
+from tank_pipe.solver import run
 
 
 GAMMA = 1.4