Remove undefined terms in parameterGroup examples

[chris-little]

Removed undefined terms narrowerThan, statisticalMeasure and statisticalPeriod from the Parameter Group examples. Add another example to the JSON-LD section.

[jonblower]

Hi @chris-little, thanks for doing this. Having thought about it, I'm wondering if we should just remove the section about statistical measures altogether for now. There's a lot of stuff that would need defining, even in the new simplified version. How about simply including some placeholder text, e.g. "ParameterGroups can also be used to gather related statistical quantities, but this will be defined in a future version of the spec"?

[jonblower]

This looks OK as far as I can tell but would benefit from a review from someone who's more familiar with how to insert new vocabularies via the JSON-LD context - perhaps @KathiSchleidt?