Processing aggregate bis

[arnaud-morvan]

Description

Include a few sentences describing the overall goals for this PR (pull request). If applicable also add screenshots.

Checklist

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• Commit messages are descriptive and explain the rationale for changes
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• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
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