renamed is_subtree() to is_treefragment() to make its intentions more clear.
treekernel notebook: fixed is_rooted_at_node(), added count_tree_fragment_occurances()
treekernel notebook: fixed common_subtrees(). tree_kernel() and tree_kernel_recursive() work correctly now.
- tree_kernel() is very slow
- tree_kernel_recursive() is much faster
minor: improved get_subtrees() docstring
test_tree module: added minimal tree fragments for testing
tree kernel refactoring: moved working functions to tree.py