Add view:azimuth and view:incidence_angle to Item properties

[matthewhanson]

Azimuth and incidence angle are provided at the Asset level only, not under Item properties, which makes them not searchable. Note that these values are slightly different per asset because of the instrument design using multiple detectors.

Furthermore, without searching, the inclusion of them at the asset level is arguably not even useful at all, since that also is a mean value over the FPA. Users who actually needed to know these real values should be accessing the full array of data from the source metadata files.

Recommendation is to calculate average azimuth and incidence_angle and put those in the Item properties, and remove them from the individual assets.

Also, the latest issues and desciptions of angles should be reviewed in https://github.com/stac-extensions/view/issues to verify that the angles used here accurately reflect the extension.