Updated best practices about applicable date-time of the mdel

[fmigneault]

🚀 Feature Request

Update the best-practices folloing discussions and allowed values (see links).

🔉 Motivation

With STAC, datetime, start_datetime and end_datetime typically represent the temporal extent applicable by the data. However, in the case of a model, the temporal extent could apply for an undefined range (i.e.: any past or future date can be processed by a given model, it does not have a temporal context limitation).

Given that STAC does not allow null values for temporal extents (at the time of writing, see discussion links), the Best Practices currently warn about recommended generic values for those open date-times.

Other ongoing discussions (OGC, GeoJSON, etc.) hint toward allowing null for similar cases. If this comes to be accepted by STAC Core specification, the best practices should be updated accordingly, which a consideration of the applied stac_version.

📡 Alternatives

📎 Additional context

• Datasets without time radiantearth/stac-spec#1268
• Representing open datetime ranges using Common Metadata radiantearth/stac-spec#1161