Add skeletal definitions for SKOS terms

[dylan-sa]

Adding skeletal definitions for SKOS terms. Omitting definitions for SHACL terms in anticipation of #1001.

Closes #934.

[dylan-sa]

Looks like the skeletal definitions are violating our prefLabel requirement. An easy solution is to add prefLabels. Another option is to tweak the SHACL and add an exception for the skeletal definitions.

[rjyounes]

Does the non-conforming label approach work here, or is some label required even if the format is not conformant?

[rjyounes]

My preference would be to tweak the shacl so that it only applies to terms in the gist namespace.

[dylan-sa]

@rjyounes @marksem -- I have tweaked the SHACL to target only classes/properties in the gist namespace.

[dylan-sa]

@rjyounes -- Thanks for review. I have pushed changes to address comments.

[rjyounes]

I have one question. Once you've clarified it, I'll approve.