added type property for catalogs and collections

[volaya]

Related Issue(s): #
radiantearth/stac-spec#971

Description:
Adds tpe property for catalogs and collections

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-io]

Codecov Report

Merging #288 (1d2b7c1) into develop (5a492ca) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #288   +/-   ##
========================================
  Coverage    93.68%   93.68%           
========================================
  Files           33       33           
  Lines         4057     4057           
========================================
  Hits          3801     3801           
  Misses         256      256           

Impacted Files	Coverage Δ
pystac/catalog.py	95.67% <ø> (ø)

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[volaya]

Before merging this, one thing to consider:

Pystac uses the 'extent' property to identify collections (see https://github.com/stac-utils/pystac/blob/develop/pystac/serialization/identify.py#L273). That should be changed to now use the type property, but doing so breaks a large number of tests, mainly because sample json files in tests do not have the type property. Those files should be updated, but before working on that, I would like a second opinion about it, to know if it is worth or not. For what I understand, the type property is now mandatory, so sample test catalogs and collections should have it.

[matthewhanson]

@volaya That's correct, this is a required property now, and Pystac should be updated to use the type to identify Collections. I think all the tests should be updated as well. I think I'd combine the identify changes and example changes into this PR.

[lossyrob]

The identify functionality identifies older version of STAC as well, so we don't want to get rid of that. It can start with a check for the property type and make decisions based on that, but if something is pre-1.0.0-RC1, it will need to fall back to it's current logic.

The STAC objects that should be updated are the main test data, but all the examples test data that are for a specific version shouldn't be changed - those test PySTAC's ability to migrate older versions to new STAC.
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[volaya]

I added a new commit with changes to the identify function, which now uses the approach suggested by @lossyrob (first try to identify based on the type property, and if that's not possible, use the previous approach).

This also fixes a problem not mentioned in #289 With that PR, Collections can now have assets, and the identify method uses the presence of the 'assets' property to identify Item objects, which will no longer be a valid strategy. No need to change anything in that PR if this one is merged.