Track merger for acts
This adds a simple (and to be improved) algorithm for merging acts tracks. It creates a new track collection obtained from the input collections. It's a real copy
After discussion with @tbold it turns out track merging is not that trivial without doing a track copy for the time being and would need developments on the ACTS side to support for that. That is mainly due to the fact the State backends has indices for identifying elements from other backends, and they would be invalidated by the merging process if we are not careful