Lazy-load ThinLayerPysces and ThinLayerCopasi imports

Refactor __init__.py to only import ThinLayerPysces and ThinLayerCopasi when they are explicitly accessed. This avoids unnecessary ImportErrors and improves module import performance by deferring dependency checks until needed.

Author: JR-1991

pyenzyme/thinlayers/__init__.py

@@ -1,13 +1,44 @@
 from .base import BaseThinLayer
 
-try:
-    from .psyces import ThinLayerPysces
-except ImportError:
-    pass
-
-try:
-    from .basico import ThinLayerCopasi
-except ImportError:
-    pass
+# Only import and show warnings if the modules are explicitly requested
+ThinLayerPysces = None
+ThinLayerCopasi = None
+
+
+def _get_pysces():
+    global ThinLayerPysces
+    if ThinLayerPysces is None:
+        try:
+            from .psyces import ThinLayerPysces as _ThinLayerPysces
+
+            ThinLayerPysces = _ThinLayerPysces
+        except ImportError as e:
+            raise ImportError(
+                f"ThinLayerPysces is not available because of missing dependencies: {e}"
+            )
+    return ThinLayerPysces
+
+
+def _get_copasi():
+    global ThinLayerCopasi
+    if ThinLayerCopasi is None:
+        try:
+            from .basico import ThinLayerCopasi as _ThinLayerCopasi
+
+            ThinLayerCopasi = _ThinLayerCopasi
+        except ImportError as e:
+            raise ImportError(
+                f"ThinLayerCopasi is not available because of missing dependencies: {e}"
+            )
+    return ThinLayerCopasi
+
+
+def __getattr__(name):
+    if name == "ThinLayerPysces":
+        return _get_pysces()
+    elif name == "ThinLayerCopasi":
+        return _get_copasi()
+    raise AttributeError(f"module '{__name__}' has no attribute '{name}'")
+
 
 __all__ = ["BaseThinLayer", "ThinLayerPysces", "ThinLayerCopasi"]