Initial values of ARMA parameters in function GetArimaFromLabel

[mafernandez]

It is suggested to change the initial values for the polynomials AR or MA into the function GetArimaFromLabel placed in stdlib/math/stat/models/mle/arima/_fun_general.tol

Just now it appears:

 Polyn ar     = GetPolynFromLabel("AR", arTxt, 0.1, period);
 Polyn ma     = GetPolynFromLabel("MA", maTxt, 0.1, period);

The change is suggested to be:

 Polyn ar     = GetPolynFromLabel("AR", arTxt, 0.1, period);
 Polyn ma     = GetPolynFromLabel("MA", maTxt, 0.2, period);

Or any other initial values, but different for AR and MA, in order to avoid the mistake of an BSR estimation when the arguments of the function ArimaStruct are constructed from GetArimaFromLabel.

thanks!

[Víctor de Buen Remiro]

(In [983]) Fixes #690

[Víctor de Buen Remiro]

(In [984]) Fixes #690