Oemetadata v2: Handling of multiple resources is not yet fully implemented

Description of the issue

THe oemetadata version 2 includes multiple resources to enable specific metadata for each resource. The metadata document itself now describes a dataset with 1 or more resources.

This requires the oemetabulder, the meta widget and all other functionality processing metadata documents to handle multiple resources. Some parts are not yet fully migrated to this new structure.

Ideas of solution

Update :

• Tag/Keyword test only handles first resource
  api/tests/test_regression/test_sync_tags_keywords_969.py

• Tag resource functionality also only handles first resource
  https://github.com/OpenEnergyPlatform/oeplatform/blob/feature-1950-migrate-oemetadata-to-v2/api/views.py#L316

• metaEditor also not yet fully supports the new structure (mostly see fix data functionality, prevents submitting metadata with multiple resources)

Additional:

• Handle date time formats (parse to required iso format)

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md