Add perturb_pmf.
Make perturb_pmf available in dit.math.
Add test for perturb_pmf.
Reorganize a bit.
Add documentation that pmf-based ops assume linearly distributed
probabilities.
Add convex_combination for pmfs.
Add jensen_shannon_divergence_pmf.
Add code for downsampling pmf to a grid.
Remove print statements.
Remove neighbors function.
Add more unit tests.
Fix perturb to work with many pmfs at once.
Add unittest for perturb on many pmfs.
Update docs.
Merge pull request #59 from chebee7i/projections

Add functions that transform pmfs.
Documentation mainly.