equivalent to scipy.preprocessing.normalize on sparse matrices
    , but lets avoid another depedency just for a small utility function

def normalize(X):
    """ equivalent to scipy.preprocessing.normalize on sparse matrices
    , but lets avoid another depedency just for a small utility function """
    X = coo_matrix(X)
    X.data = X.data / sqrt(bincount(X.row, X.data ** 2))[X.row]
    return X