Added PIL graphics - 5-10 times faster than MPL
move adaptColor into GraphicsLayer
change scatter size arg to s like MPL
move clf, get_cmap up into GraphicsLayer
added boolean "train" arg to ddqn step() to make running faster in player / editor
add plotTreeObs - not finished.
improved pixel scaling in editor (I hope)
Made the agent background steps update a progress bar.

new generators that take agents_handles as extra input and return the agent's initial position, direction and target

New notebook Editor2.ipynb.

cell_neighbours_valid

Add deadend logic for start, end of mouse stroke.
Use remove_deadends in set_transition for 3 cell transitions.

in Grid4Transition.
Add has_deadend and remove_deadends() in RailEnvTransitions.
(should they be in RailEnv or Grid4...?)

in GridTransitionMap.

add cell_neighbours_valid, is_cell_valid to transition_map
add invalid cell detection to editor and rendertools