'Number of non-zero entries, equivalent to
np.count_nonzero(a.toarray())
Unlike getnnz() and the nnz property, which return the number of stored
entries (the length of the data attribute), this method counts the
actual number of non-zero entries in data.'

def count_nonzero(self):

    raise NotImplementedError(('count_nonzero    not    implemented    for    %s.' % self.__class__.__name__))
