Compute p@10 using 0 == irrel, 1+ == rel, so that P@10 is in the expected range [0..1]

[david-fisher]

P@10 when used with multivalued judgements should still produce a score between 0 and 1.

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• [] Bug fix (non-breaking change which fixes an issue)
• [] Improvement (non-breaking change which improves existing functionality)
• [] New feature (non-breaking change which adds new functionality)
• [] Breaking change (fix or feature that would cause existing functionality to change)

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• My code follows the code style of this project.
• [] My change requires a change to the documentation.
• [] I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• [] I have added tests to cover my changes.
• [] All new and existing tests passed.