added static assert + minor docs

python/src/bind_Cluster.hpp

@@ -91,6 +91,7 @@ void reduce_to_3x3(py::module &m) {
         py::return_value_policy::move, py::arg("cluster"), R"doc(
         Return the 3x3 block around the cluster's center index.
 
+        The input cluster dimensions must both be odd.
         The input coordinates are preserved and output data is stored in
         row-major order.
         )doc");

python/src/bind_ClusterVector.hpp

@@ -225,6 +225,7 @@ void define_3x3_reduction(py::module &m) {
         },
         R"doc(
         Reduce every cluster to the 3x3 block around its center index.
+        The input cluster dimensions must both be odd.
         Returns a new ClusterVector; cluster order, coordinates, and frame
         number are preserved.
         )doc",