Add the support of relational EMR data for healthcare application

[chrishkchris]

One of the target in the PANDA (SINGA-easy) project is to support of relational EMR data for helathcare application.

Some reference papers:

1. Zheng et al. Capturing Feature-Level Irregularity in Disease Progression Modeling, CIKM' 17 https://dl.acm.org/doi/abs/10.1145/3132847.3132944
2. Shickel et al. Deep EHR: A Survey of Recent Advances in Deep Learning Techniques for Electronic Health Record (EHR) Analysis. arXiv:1706.03446v2
   https://arxiv.org/abs/1706.03446

A real-world EMR dataset is MIMIC dataset
https://mimic.physionet.org/gettingstarted/access/