Fix computation of SuccessionAsUsage.getSourceFeature when using implicit source

[adaussy]

In some case the implicit source of a SuccessionAsUsage is not the direct previous feature.

For example in the following use case:

 action def ActionDef1 {
                    action ax2;
                    action ax1;
                    then ax2;
                    then decide;
                }

The previous feature of the last SuccessionAsUsage (ax1 -> decide) would be the previous SuccessionAsUsage (ax1 -> ax2) . The computation of the previous feature should exclude any 'TransitionUsage' and 'SuccessionAsUsage'.