refactor: organize src modules by category

src/cylinder/__init__.py

@@ -0,0 +1,17 @@
+"""High-pressure gas cylinder models."""
+
+from cylinder.gas_cylinder import (
+    DEFAULT_FLUID,
+    DEFAULT_P_INIT,
+    DEFAULT_T_INIT,
+    DEFAULT_VOLUME,
+    HighPressureGasCylinder,
+)
+
+__all__ = [
+    "DEFAULT_FLUID",
+    "DEFAULT_P_INIT",
+    "DEFAULT_T_INIT",
+    "DEFAULT_VOLUME",
+    "HighPressureGasCylinder",
+]