Corrected benchmark function to use the columns.

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Parametrise tests consistently.

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update docs for image anchor

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Refactored column name to index conversion. `fast` is now obsolete as an argument.

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branch : 1.8
Merged in TWAC/openpyxl/1.8 (pull request #64)

update docs for image anchor

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branch : 1.8
Minor refactoring and more parametrizing.

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branch : 1.8
Remove marked test as related bug is invalid.
Updated docs to cover formulae.

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branch : 1.8