refactor: organize src modules by category

tests/test_gas_cylinder.py

@@ -1,7 +1,7 @@
 import numpy as np
 import pytest
 
-from gas_cylinder import (
+from cylinder.gas_cylinder import (
     DEFAULT_FLUID,
     DEFAULT_P_INIT,
     DEFAULT_T_INIT,

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

tests/test_pipe.py

@@ -1,7 +1,7 @@
 # tests/test_pipe.py
 import numpy as np
 import pytest
-from pipe import Pipe
+from tank_pipe.pipe import Pipe
 
 
 GAMMA = 1.4

tests/test_riemann.py

@@ -1,6 +1,6 @@
 # tests/test_riemann.py
 import numpy as np
-from riemann import hll_flux
+from tank_pipe.riemann import hll_flux
 
 
 GAMMA = 1.4

tests/test_tank.py

@@ -1,7 +1,7 @@
 # tests/test_tank.py
 import numpy as np
 import pytest
-from tank import Tank
+from tank_pipe.tank import Tank
 
 
 GAMMA = 1.4