Twined

Summary
Reverts octue/twined104.

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Contents ([105](https://github.com/octue/twined/pull/105))

Reversions
- Revert "Add output location strand"

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Summary
Add the ability to provide an output location in `twine.json` - this is a path to a Google Cloud Storage directory where all output datasets will be uploaded to.

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Contents ([104](https://github.com/octue/twined/pull/104))

New features
- Add output location strand

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Contents ([103](https://github.com/octue/twined/pull/103))

Enhancements
- Allow datasets in manifest strands to be optional

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Summary
Allow a manifest's datasets to be specified as paths, allowing metadata to be gathered dynamically from these datasets instead of hard-coded in a manifest serialisation.

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Contents ([101](https://github.com/octue/twined/pull/101))
**IMPORTANT:** There are 2 breaking changes.

Enhancements
- **BREAKING CHANGE:** Make `Twine.available_strands` a set instead of a tuple
- Update manifest schema to allow path-only datasets
- Raise error if a dataset specified in a manifest strand of the twine is missing from the corresponding manifest
- Add `Twine.available_manifest_strands` property

Refactoring
- **BREAKING CHANGE:** Move dataset files tag checking into `octue-sdk-python` at `octue.runner.Runner._validate_dataset_file_tags`

Refactoring
- Move manifest and twine migrations into `migrations` subpackage
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Summary
Use the latest JSON metaschema and switch to HTTPS to access it.

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Contents ([100](https://github.com/octue/twined/pull/100))

Enhancements
- Use latest JSON metaschema `2020-12` and access it via HTTPS

Fixes
- Fix labels ref in manifest schema

Dependencies
- Upgrade to `jsonschema==4.4.0`

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Summary
Remove the credentials environment variable name from the children schema. The default `GOOGLE_APPLICATION_CREDENTIALS` is the only name now used in `octue-sdk-python`.

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Contents ([98](https://github.com/octue/twined/pull/98))

Enhancements
- Remove the credentials environment variable name from the children schema
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