Define mapping to a disease association file (DAF)

[cmungall]

Similar to #67

We want to provide either examples of how to represent model organism or model system disease model associations, or provide a mapping to a simple DAF format (or series of DAF formats for different purposes).

Just like a PAF, the entity being annotated could be anything from a gene to allele to a genotype + environment. There should be evidence for each association. The association is always to a disease, this can probably be assumed to be a pre-composed ID from a suitable ontology (but there may be cases for post-composition, e.g. some cancers?)

It's not clear whether DAF needs to be an official TSV serialization, but providing a mapping will add clarity ad be useful for delivery. E.g. users would want to download a TSV for analysis. Some users may want to submit DAFs?