12358 parse metadata (rebased onto dev_5_0)



This is the same as gh-2583 but rebased onto dev_5_0.

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See https://trac.openmicroscopy.org.uk/ome/ticket/12358#comment:1

DV metadata objects were expecting to be processed via the `toString()` method. This no longer holds. Some for of API addition is needed in order to make this more general.

/cc @melissalinkert @mtbc