HML Spec: For all elements that allow a URI attribute, do we need an additional 'format' or 'content' attribute to define what is at the URI endpoint?

[dvaliga]

[mpresteg]

Related to issue #11

For the the <raw-reads> element I think this might be necessary.

For <glstring> I think the use case is clear, and is ok with URIs-without-content-type, as those are used as identifiers by the gl-service.

For other URI properties, see nmdp-bioinformatics/hml#14