Update Ecal clustering with the CLUE algorithm

[bryngemark]

Lund summer student @Lysarina has implemented the CLUE algorithm for the LDMX Ecal, with good results (1 cluster per shower, in most cases).

Describe the solution you'd like
Now I'd like Ella's code to be merged into trunk for proper contributions attribution and to set the stage for future development.

Describe alternatives you've considered
Staying on a dev branch until long after the person doing the work has stopped working on it.