Mycophenolate mofetil Recruiting Phase 2 Trials for Refractory B-Cell Non-Hodgkin Lymphoma / Refractory Burkitt Lymphoma / Refractory Diffuse Large B Cell Lymphoma / Post Transplant Lymphoproliferative Disorder / Recurrent Adult Diffuse Large Cell Lymphoma / Recurrent B-Cell Non-Hodgkin Lymphoma / Recurrent Burkitt Lymphoma Treatment