Support master branch of `ndarray`

[duskmoon314]

I want to use both methods in ndarray's master branch and this crate. But this crate only supports specific ndarray (0.15.0 for now), which is incompatible with the master branch of ndarray.

I wonder if there is a way to use ndarray-stats and ndarray's master branch instead of cloning and changing the dependencies to git URLs.

Maybe merging ndarray-stats to ndarray and forming a workspace is a solution. In such a way, multiple locations could be used.