Add more unit tests for RVFunctions.
Update docstring.
Validate arguments to simplex_grid()
Refactor downsample(). Improve projections()

Changes:
    1) Update API to match simplex_grid().
    2) Lower-level function now takes op to determine min/max route.
    3) projections() now works with multiple pmfs.
    4) projections() can find other nearby grid points.