Mandatory fields: Change to "importance level"?

A few reviewers have proposed dropping the concept of mandatory field, this being a checklist rather than a schema.

Example suggestion from @sveinugu:

_The “Mandatory” field seems overly strict for some fields. Several of the fields that we suggest to leave out or include as optional are specified as mandatory: design_description, instrument, sequencing_protocol.

Some of these want to leave out due to the fact that we aim for a higher level set of metadata fields that refer back to original metadata for more details (when such are available). Also, since we want to make use of our schema for historical sources, some of these fields might simply not be available.

For use of the Experiments Metadata in other contexts, such as to use them for checklists for data deposition, the decision whether fields should be required necessarily follow different arguments. To make GA4GH Experiments Metadata as generally useful as possible, we could perhaps loosen the requirements, or introduce a third, middle level of compliance, say “recommended” fields (although, I guess they are all recommended...)_

Start by reviewing how the Experiments Metadata standard defines mandatory fields, especially design_description, instrument, and sequencing_protocol. Compare checklist and schema use cases, then document a decided compliance model and update the affected specification wherever those field requirements are defined.

Written by the indexing model from the issue text.