fixed #1: sorted rhs of production rules for later comparison
fixed docstring of get_production_rules()
added treekernel notebook from alt-mulig repo
tree kernel: added get_subtrees(). removed broken is_subtree()/get_syntax_subtrees()
updated treekernel notebook
tree kernel: rewrote is_subtree(), refactored get_subtrees()
renamed tree_kernel module to tree
fixed module call / var name in get_subtrees()
tree: added possibility to extract production rules from node attributes
treekernel notebook: examples, dot graphs, find_all_common_subtrees_bruteforce()