add 'definition' property to Metric.json

[zopalmer14]

Added a definition property to Metric.json as it is listed as a recommended property in the documentation but was missing from the jsonschema definition.

I would appreciate a review of the $id field value. I decided to match the approach/values used by the other two Metric properties but am a little confused about what URI/IRI is appropriate to use for the $id when there may be multiple options. For instance, the DOI documentation lists the URI as https://www.w3.org/TR/skos-reference/#definition; however, generally speaking, the jsonschema $id values don't seem to map to those. Additionally, as the definition property belongs to the SKOS vocabulary/namespace, other files like Concept.json include it and use an $id value of http://www.w3.org/2004/02/skos/core#definition.

DOI DCAT-US 3.0 Documentation: Metric - definition

Should resolve: #66

[zopalmer14]

I just now realized that, since the cardinality of Metric.definition is 0..n, the property should accept either a null value or an array of strings rather than a single string as I specified in my changes. I would make an additional commit to change the PR, but I thought this could be a point of discussion as the more I dive into the codebase/jsonschema, the more I notice discrepancies between the cardinality of properties in the documentation and the jsonschema definitions.

Additionally, perhaps it is worth discussing whether it makes sense for there to be multiple definitions for a given Metric.

[jbrown-xentity]

Actually, this looks right to me.
I think the documentation is wrong. I don't see any value in having a metric object that has a definition field that allows for multiple definitions. The definition should be singular, and self contained.