add recursive clustering and skeletonization

Author: alexfikl

pytential/linalg/cluster.py

@@ -232,7 +232,7 @@ def make_block(i: int, j: int):
 
 # {{{ cluster generation
 
-def _build_binary_ish_tree_from_starts(starts: np.ndarray) -> ClusterTree:
+def _build_binary_ish_tree_from_indices(starts: np.ndarray) -> ClusterTree:
     partition_box_ids = np.arange(starts.size - 1)
 
     box_ids = partition_box_ids
@@ -325,7 +325,7 @@ def partition_by_nodes(
         starts = np.linspace(0, discr.ndofs, nclusters + 1, dtype=np.int64)
         assert starts[-1] == discr.ndofs
 
-        ctree = _build_binary_ish_tree_from_starts(starts)
+        ctree = _build_binary_ish_tree_from_indices(starts)
 
     from pytential.linalg import make_index_list
     return make_index_list(indices, starts=starts), ctree