Recomputing neighbors leaves a stale mapping matrix #91

compute_neighbors and load_precomputed_distances replace self.knn but don't reset self._mapping_operator. map() only rebuilds the operator when it is None, so this silently uses weights from the old graph:

cm.map(obs_keys="celltype")
cm.compute_neighbors(n_neighbors=50)  # new graph
cm.map(obs_keys="celltype")         # still uses the old mapping matrix

• cellmapper/src/cellmapper/model/cellmapper.py

  Line 227 in f68b451

  self.knn = knn

• cellmapper/src/cellmapper/model/cellmapper.py

  Line 657 in f68b451

  self.knn = Kernel.from_distances(distances_matrix, remove_last_neighbor)

• cellmapper/src/cellmapper/model/cellmapper.py

  Lines 580 to 581 in f68b451

  	if self._mapping_operator is None:
  	self.compute_mapping_matrix(kernel_method=kernel_method, symmetrize=symmetrize, self_edges=self_edges)

Expected: setting a new knn invalidates _mapping_operator (e.g. reset it in both places, or make knn a property whose setter does so).