DV extended metadata

Some work has been done in https://github.com/DiamondLightSource/PySIMRecon/tree/get-extended-header-working but this needs to be checked against DV files from softworx. Also probably could be improved.

Start by reviewing the existing work in the get-extended-header-working branch. Check that implementation against DV files from softworx, then identify and document any improvements needed; done means extended metadata works correctly for those files.

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