http://papers.www2017.com.au.s3-website-ap-southeast-2.amazonaws.com/proceedings/p765.pdf
Modified from https://github.com/tianlinyang/DKVMN to work with files directly downloaded from https://sites.google.com/site/assistmentsdata/home/assistment-2009-2010-data/skill-builder-data-2009-2010
http://papers.www2017.com.au.s3-website-ap-southeast-2.amazonaws.com/proceedings/p765.pdf