Add bands

[gadomski]

Related Issue(s):

• Implementation of RFC: A common construct to replace `raster:bands` and `eo:bands` radiantearth/stac-spec#1213 (comment)
• Closes Add Asset.bands #1225

Description:

WIP until STAC 1.1 is out w/ bands. Might need more fields on bands.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• 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]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
pystac/__init__.py	94.73% <100.00%> (+0.09%)	⬆️
pystac/asset.py	97.43% <100.00%> (+0.40%)	⬆️
pystac/collection.py	93.31% <100.00%> (+0.35%)	⬆️
pystac/item.py	94.37% <100.00%> (+0.39%)	⬆️
pystac/band.py	81.81% <81.81%> (ø)

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