Forward electron clusters make them also have constituent links

Mentioning @jcatmore .

In principle for forwardElectronCluster we do not stricly need constituentLinks unlike egammaClusters which are so called superClusters.

Anyhow, since for some reason it seems that code paths outside egamma somehow lead to conditional decorations, and since they can be defined as pointing back to the CaloCalTopoCluster we copy. And in principle prb is easier if we have them always there. and assuming that the cost in disk should be small .

Lets do it from inside e/gamma for all of them...