Clarify eopf:type + relationship with collections

[jdries]

Regarding eopf:type: producttype is present in older catalog standards, but STAC did not include it as a standard property, which I consider a very good decision. In 99% of cases, a collection should contain a single producttype, because collections with multiple product types are often too heterogenous, so the collection level metadata becomes so generic that it's effectively useless.

I've seen examples where the collection is simply 'sentinel-3', and the wide variety of sentinel-3 product types is contained in that collection. Hence it becomes impossible to create sensible collection level metadata.

Of course, I don't object to having the propery, as it can also be set to the same value for all items in a given collection, although I'm afraid that it will invite catalog providers to create overly heterogenous collections. Hence it requires a clear warning in the documentation of this extension if we want to avoid that.

[cbouzina]

Final decisions taken in agreement with ESA and Copernicus