[WIP] account for extensions in collections

[simonkassel]

Figure out how we we handle extension properties in collections.

I.e. if a collection includes extended items like EOItems, how should the collection store the properties specific to the EO extension.

[simonkassel]

Currently, the collection includes eo properties if they are uniform across all eo items and Null values if they are not.

this comment complicates that matter radiantearth/stac-spec#413 (comment). Should EO items with differing properties even be allowed to be in the same collection?

[lossyrob]

This functionality was put in place via another means - an Item merges in the Collection common properties when loading.