Add source_for relation type for forward provenance

[emmanuelmathot]

Introduce the source_for relation type to support forward provenance, allowing a source item to link to derived items. This addition enhances the documentation and changelog to reflect the new relation type, which is the inverse of derived_from.

Fixes #27

[m-mohr]

Generally a good idea, I'm just trying to remember whether I have seen this before (which I think) and whether it was using a different relation type. Trying to find it again, maybe worth checking with OGC (Provenance?) as well.

(Side note: It's a bit of a pity that we use _ here and in derived_from while in (all?) other relations we use -)

[emmanuelmathot]

I checked OGC: the OGC link relation register and IANA have no forward-provenance relation. OGC Testbed 20 (24-036) uses W3C PROV, whose inverse of wasDerivedFrom is hadDerivation (PROV-O inverses note). source_for comes from stac-spec#1179. I kept the underscore to pair with derived_from, but I'm open to derivation/had-derivation if you prefer PROV alignment.

[m-mohr]

No, I have no strong preference. We can use source_for.

[fmigneault]

Just saw this a bit late.

How about used_by (i.e.: as for prov:wasUsedBy), the recommended inverse of used (https://www.w3.org/TR/prov-o/#inverse-names-table)?

While a derived_from (prov:wasDerivedFrom) can be used to reflect an entity transformed to another, is it safe to say that, inversely a "source" was employed without providing anything regarding what what it was "used by"?

It is a bit confusing because the https://github.com/stac-extensions/processing#relation-types table employs specifically "that was used as input data" for the derived_from description, but then doesn't employ used for the inverse. There also a mixture of incorrect PROV Entity/Activity concepts relationships happening.

It would make a lot of sense to employ used_by when a processing:software or processing:expression (the processing Activity) is involved

     ┌──rel:"used_by"─┐ -------------┐ (same STAC)
     │                ▼              ▼ 
rel:"source"  -> processing:* -> derived 
[STAC Entity]    [Activity]      [STAC Entity]
     ▲                               │
     └────── rel:"derived_from" ─────┘

Since the processing:* are annotated within the same document of the derived STAC Entity resulting from applying that processing activity, that would work. There would only be a requirement that the rel: used_by STAC being referenced should contain a processing:* definition.

[m-mohr]

We haven't released yet, feel free to open a PR.