'Returns a matrix of ones.
Args
rows : rows of the matrix
cols : cols of the matrix (if None, cols=rows)
kwargs
cls : class of the returned matrix'

@classmethod
def ones(kls, rows, cols=None, **kwargs):

    if (cols is None):
        cols = rows
    klass = kwargs.get('cls', kls)
    (rows, cols) = (as_int(rows), as_int(cols))
    return klass._eval_ones(rows, cols)
