Outcomes of Allogeneic Hematopoietic Cell Transplantation (allo-HCT) for Adults with Primary Mediastinal B Cells Lymphoma (PMBCL): A Retrospective Study on Behalf of the Société Francophone De Greffe De Moelle Et De Thérapie Cellulaire (SFGM-TC) and the Lymphoma Study Association (LYSA) Group