'Convenience function to create an ArmaProcess from the results
of an ARMA estimation
Parameters
model_results : ARMAResults instance
A fitted model
nobs : int, optional
If None, nobs is taken from the results'

@classmethod
def from_estimation(cls, model_results, nobs=None):

    arcoefs = model_results.arparams
    macoefs = model_results.maparams
    nobs = (nobs or model_results.nobs)
    return cls(np.r_[(1, (- arcoefs))], np.r_[(1, macoefs)], nobs=nobs)
