Base class for BPR-MF plus attribute-to-factor mapping
Literature: Zeno Gantner, Lucas Drumond, Christoph Freudenthaler, Steffen Rendle, Lars Schmidt-Thieme: Learning Attribute-to-Feature Mappings for Cold-Start Recommendations. ICDM 2011. http://www.ismll.uni-hildesheim.de/pub/pdfs/Gantner_et_al2010Mapping.pdf