Revert "ch.py (clear_cache): now do this so that less nodes are visited. Number of nodes visited is unit tested also in test_ch.py."

This reverts commit 46bb2dd15a935e158c5de4553ec98aef05938adc.
Revert "ch.py: Added "one-parent" heuristic, removed "cache clear means parents cache clear" heuristic, and added memory usage visualization in Ch.show_tree()"

This reverts commit b6308682470d7a7a98a7bfb6e9ef8c07ad66068a.