replace np.alen with len

[RyotaUshio]

Thank you for the awesome paper and this implementation.

np.alen was deprecated in numpy 1.18.0 (https://numpy.org/doc/stable/release/1.18.0-notes.html#deprecate-np-alen) and has already been removed in the current version of numpy. This PR replaces all occurences of np.alen with the built-in len function so that this library keeps working.