Describe what happens

When building constraints we do something like:

        cns   = zfit.constraint.GaussianConstraint(
            params         = l_par, 
            observation = l_obs_par,
            cov                 = cov)

where the stuff starting with l_ is meant to be a list. This should allow for an n-dimensional Gaussian cosntraint where each element of l_par gets constrained by the corresponding element in l_obs_par and the pairing goes to the cov matrix. All this assuming that the ordering is the same.

However I mistakenly used a set for the l_par. This means that, given that sets are not ordered, the constraints get swapped between components.

In a place like this convert_to_container could raise an exception if an unnordered container is passed by mistake.