Make vars optional in 'printf' fields?

[dosumis]

Patterns sometimes need to spec a constant filler for an axiom. e.g. patterns in GO require a namespace to be specified for all implementations of the patterns. We can also imagine cases where we want to spec a constant SubClassOf axiom (=> consistent GCI) - although this may be a slightly dangerous approach.

In the current spec we could support this by forcing spec of an empty vars list in a pattern. But a more compact solution is simply to allow the vars field to be optional.

=>

namespace:
   text: biological_process

While dropping the text field altogether would look cleaner in this case, this is harder to build on top of the core spec, where we still need an annotationProperty field:

annotations:
   -
       annotationProperty: obo_namespace
       text: biological_process