Add simplex_grid and simplex_index

[oyamad]

From my old code https://github.com/oyamad/simplex_grid.
To be used in game_theory.

[coveralls]

Coverage decreased (-0.6%) to 93.033% when pulling 37ff345 on simplex_grid into ecbf2b2 on master.

[mmcky]

@oyamad do you know why coverage has decreased with this PR? It looks like there are tests that should cover the additional routines.

[oyamad]

It is because of the @jit decorator. It seems that a jitted function is not covered even without nopython=True.

[mmcky]

ah - I should add the simple case for .coveragerc

Currently we have:

    # Don't count Numba nopython=True jit functions
    @jit\(.*nopython=True
    @njit

[mmcky]

is there a reason not to use njit?

[oyamad]

Because num_compositions is not jitted which uses scipy.misc.comb.

I could have implemented scipy.misc.comb with Numba, but I decided not to because it is not really necessary (@jit just works fine) and it is a nontrivial job to handle overflow.

[coveralls]

Coverage increased (+0.3%) to 93.924% when pulling 4b527f0 on simplex_grid into ecbf2b2 on master.