Support multidimensionality

[pavelkomarov]

In numpy we can often feed a method a multidimensional array and an axis parameter that specifies with direction we're operating along. The methods in this package are designed to work on univariate data streams, but it would be nice to generalize them to work with arrays of more dimensions.

Alternatively we could keep all methods univariate and force users to do something like apply_along_axis, however this loops up in Python, not taking advantage of speedups from numpy that may be possible with calls like fft, which push that loop down in to compiled C.