scale covariance K such that it explains unit variance
    trace_method: standardize to unit trace (deafault: True)

def scale_K(K, verbose=False,trace_method=True):
    """scale covariance K such that it explains unit variance
    trace_method: standardize to unit trace (deafault: True)
    """
    if trace_method:
        scalar=1.0/(K.diagonal().mean())
    else:
        c = SP.sum((SP.eye(len(K)) - (1.0 / len(K)) * SP.ones(K.shape)) * SP.array(K))
        scalar = (len(K) - 1) / c
    if verbose:
        print(('Kinship scaled by: %0.4f' % scalar))
    K = K * scalar
    return K