update metadata extractors in extractors core

[tcnichol]

Issue is this:
clowder-framework/extractors-core#27

Adding a new extractor to v2 will allow for better testing of extractor features and we can add some as default extractors in a yml file.

[tcnichol]

Updating this will require some additional work.

The context field in metadata needs to be a list that can contain URL or dictionary to match json-ld specs. That will be done in clowderv2 with another pull request.

Pyclowder2 will need to be changed to match this as well.

[ddey2]

What's the status?