- Fix relative imports in base of is a package or not
- Fix not working notebooks
- Add notebooks to packages sources

Used tool 2to3 to finish the porting to Python 3 plus some fixes by
hand.

These functions, which I had to implement in Python 2.2, are now
available in standard Python.

use “raise NotImplementedError” for abstract methods

Remove astar row from compare_graph_searchers table because it's sensitive to trivial changes. Needs real tests instead.

 * graph_search missed deduping nodes in the frontier
 * astar_search didn't use 3rd-edition pseudocode
 * InstrumentedProblem didn't forward the path_cost method,
   so all the doctest results on instrumented cost-sensitive
   problems were wrong

Also filled in breadth_first_search and per-node memoization 
of h (to fulfill the doc comment saying that was done).

There's a remaining problem that sometimes trivial refactorings
change the amount of search that is done, slightly, which should
not happen since the code is supposed to be deterministic.

Don't compare strings with 'is not'. (Broke things in pypy.) Also show Struct members in sorted order so doctests work in pypy.

Agent.__init__ now takes a program parameter. A few more nonlocal-state-variable problems found and fixed. One agent program rewritten as a callable instance since it uses both state variables and abstract methods.

Moved agent-program definition back into the constructors to get rid of all the copying between parameters, instance vars, and nonlocals. Fixed nonlocal state-var bug in ReflexAgentWithState.

Some genetic_search() fixes: states were lists but had .mutate() called on them; pmut was probability of *not* mutating; initial population nonrandom; bad fitness function.