Egamma forward builder cookie cut addative

Currently when we cookie cut forward clusters we copy the original cluster and remove cells based on their proximity to the hottest cell.

This is computationally inefficient as the cell links container gets copied every time we want to remove a cell.

This MR is a first pass at using the methodology used in egammaSuperClusterBuilderBase where the cell links container starts empty and is filled with cells passing the selection criteria. This eliminates expensive copys of the entire container and brings egammaForwardBuilder closer to egammaSuperClusterBuilder reducing the complexity of future re-factorisation of both classes.