Refactor gth_solve.py; add documentation of gth_solve and graph_tools

1.
In gth_solve.py I replaced sum + list comprehension with dot + indexed arrays. The current version gets about 10% faster in some best case.
(I also corrected a typo in test_gth_solve_py.)

2.
I realized that `gth_solve` and `DiGraph` (in graph_tools.py) do not appear on [readthedocs](https://quanteconpy.readthedocs.org).
To let them show up, I added graph_tools and gth_solve to docs/source/tools.rst. Do I also need to create the files "graph_tools.rst" and "gth_solve.rst"?