Description

Changed existing endpoints
instruments/ and urns/?{urn}
to
instruments/ and instruments/urn/{urn}

Added endpoints:
measures/ and measures/urn/{urn}
requirements/ and requirements/urn/{urn}

Resolves #341

Checklist

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• I have tested the changes locally and verified that they work as expected.
• I have followed the project's coding conventions and style guidelines.
• I have rebased my branch onto the latest commit of the main branch.
• I have squashed or reorganized my commits into logical units.
• I have read, understood and agree to the Developer Certificate of Origin, which this project utilizes.