Add Numba jit version of scipy.special.comb

[oyamad]

1. Add comb_jit, Numba jit version of scipy.special.comb
   Returns 0 if the outcome exceeds the maximum value of np.intp
2. Add num_compositions_jit which uses comb_jit
3. Compile simplex_grid in nopython mode using num_compositions_jit

[coveralls]

Coverage increased (+0.04%) to 94.836% when pulling 276df50 on comb_jit into 296e595 on master.

[mmcky]

thanks @oyamad. Sorry I have been off grid for a couple of weeks. I will merge this later today.

[oyamad]

@mmcky No problem at all!
(Upon merge, it would be nice if you merge the commits as they are without squashing; these commits are logically separate.)

[mmcky]

Thanks @oyamad for letting me know re: squash. Does this PR rely on #375 to be merged first?

[oyamad]

@mmcky These PRs are independent. Either PR can be merged first.

[mmcky]

@oyamad when you get a chance would you mind reviewing this merge conflict. Thanks.

[oyamad]

@mmcky Rebased on master. Thanks.

[coveralls]

Coverage increased (+0.02%) to 94.843% when pulling 0c35d78 on comb_jit into 828aaf2 on master.