Use HDF5 array structures for ArrayValue

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In HDF5 (and to a lesser extent JSON/YAML) ArrayValueRow doesn't make sense as we should really be using the in-built array structures (I still personally think that even in XML it is a bit of overkill and a space delimited list of values would work okay).

Also, I think we can probably drop the ExternalArrayValue element, which always felt like a hack. Maybe in it's place we could have a NamedValue "document-level" type that could be referenced like other elements.

Sparse arrays would still be required (see #87), so probably a new type, SparseArrayValue, should be introduced to specifically add support for sparse arrays.

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Adding a separate NamedValue type could probably be avoided by Andrew's suggestion #39