Change the way find_neighbor_ methods work when flatten is False

[jgostick]

At present flatten=False it returns a list of lists, which is not very numerically efficient. Instead is should return as CSR-style, so instead of [ [0, 2, 4], [4, 6]] it returns ([0, 2, 4, 4, 6], [0, 3]) where the second element of the tuple is indices into the first element where each new row starts. You could get each set of neighbors with ind, ptr = <crs-style data> so ind[ptr[0]:ptr[1]]

[jgostick]

This would really hurt readability. Maybe a better idea would be to add an additional option or function to get this CSR style if needed for performance?