add io to support general stru formats

Author: jieli-matrix

python/pyabacus/src/pyabacus/__init__.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-__submodules__ = ["ModuleBase", "ModuleNAO", "hsolver", "Cell", "IntegralCalculator"]
+__submodules__ = ["ModuleBase", "ModuleNAO", "hsolver", "Cell", "IntegralCalculator", "io"]
 __all__ = list(__submodules__)
 
 def __getattr__(attr):
@@ -16,5 +16,8 @@ def __getattr__(attr):
     elif attr == "Cell":
         from .cell import Cell
         return Cell
+    elif attr == "io":
+        import pyabacus.io as io
+        return io
     else:
         raise AttributeError(f"module {__name__} has no attribute {attr}")