This implements sparse arrays of arbitrary dimension on top of numpy and
scipy.sparse. It generalizes the scipy.sparse.coo_matrix and
scipy.sparse.dok_matrix layouts, but extends beyond just rows and columns to an
arbitrary number of dimensions.

Additionally, this project maintains compatibility with the numpy.ndarray
interface rather than the numpy.matrix interface used in scipy.sparse

These differences make this project useful in certain situations where
scipy.sparse matrices are not well suited, but it should not be considered a
full replacement. The data structures in pydata/sparse complement and can be
used in conjunction with the fast linear algebra routines inside scipy.sparse. A
format conversion or copy may be required.