Add warnings to tree-related stuff

[Luke-Poeppel]

I'm not [currently] interested in maintaining the tree stuff in decitala, so just add some warnings to the searches (search.rolling_tree_search(...)) that suggest using the hash-table approach instead. Something like:

The tree-based search methods are not currently maintained. We recommend using search.rolling_hash_search with a hash_table.FragmentHashTable instead. This approach is much faster.