Consider changes in usedBy

[libremente]

As of now, it is not clear if the usedBy key should contain also the name of the PA which owns the copyright or not. Probably it should be clearer.

[johanlinaker]

In addition, I would propose either

• removing usedBy and enabling a meta-standard which each national catalouge can maintain, or
• having usedBy per nation, e.g., swe/usedBy as with descriptions and features
  Let me know if I should create a new issue for this.

[lsmith77]

I agree with removing, see also #187

[bfabio]

Proposal in #187.