MultiDimensionalWeighted and GSL

[Peter]

One of the design criteria for weighted implementations is that passing unity weights should result in the same as the non-weighted counterpart. In MultiDimensionalWeighted (and thereby inherited to PolynomialWeighted) this leads to problem most likely because of GSL. It seems GSL has implemented the estimation of the parameter covariance matrix in a fashion not fullfilling the criterion above.

See here for GSL discussion ​http://www.mail-archive.com/bug-gsl@gnu.org/msg00093.html

Should we skip the criterion here and just follow GSL or should we try to hack around it?

needed for #161

[Peter]

I checked out GSL from cvs repoitory and the file in question was not changed since gsl-1.8, so using a updated version of GSL would not solve this issue.

I think I will copy the code from GSL (into MultiDimensionalWeighted?.cc) and add the estimation of s2 that is needed to calculate the covariance of the parameters in satisfying fashion.

[Peter]

I realised, it might be possible that the only thing needed is to multiply covariance_ with an estimate of s2. This correction could be done after the call to the GSL function as s2 is trivial to estimate (more or less the weighted sum of squared residuals).

I try this and add a add a test checking that unity weights is equivalent to using the non-weighted version.

[Peter]

(In [739]) fixes #178 and removes a bug in MultiDimensional?