migrate - Create Credit State Migration (Transition) Matrices

Tools to help convert credit risk data at two timepoints into traditional credit state migration (aka, "transition") matrices. At a higher level, 'migrate' is intended to help an analyst understand how risk moved in their credit portfolio over a time interval. References to this methodology include: 1. Schuermann, T. (2008) <doi:10.1002/9780470061596.risk0409>. 2. Perederiy, V. (2017) <doi:10.48550/arXiv.1708.00062>.