Support for Interface cluster sources

• Add cluster handling to ModelLoader.Interface
• Refactor ClusterRuptureSet to contain list of generic RuptureSet

changed milestone to %nshm-conus Development

added 2023 ERF labels

assigned to @pmpowers

mentioned in issue ghsc/nshmp/nshms/nshm-conus#23 (closed)

mentioned in merge request !284 (merged)

closed with merge request !284 (merged)

mentioned in commit 09dd0a4d

changed milestone to %Finalize draft NSHM 2023

changed milestone to %nshm-conus Development