Fix fresnel sampling

Wrong factor 1/2 in code, due to assumed padding in reference paper.

src/hotopy/holo/__init__.py

@@ -32,11 +32,13 @@ Transport of Intensity (TIE) used for laboratory X-ray sources.
     GeneralizedPaganin
 
 
-Forward propagation
--------------------
+Propagation submodule (:mod:`.propagation`)
+---------------------------------
 .. autosummary::
     :toctree: generated/
 
+    propagation
+
     FresnelTFPropagator
     simulate_hologram