Improve JSON-LD output for itemlists

[alastair]

Some improvements and bug fixes for json-ld output

• Some nodes don't serialise to json-ld #193 - only serialise datetime fields to iso8601 if the date field is non-null
• When serialising a DefinedTermSet, ensure that its @type includes oa:Motivation if the DefinedTermSet is being used as a motivation
• when we move additionalType values to @type, delete additionalType afterwards
• Make ItemList serialisation more detailed - details below.

Within schema.org, it is possible to have:

ItemList
   itemListElement -> directly linking to another item in the CE

or

ItemList
   itemListElement (a ListItem)
       item -> this is a link to another item in the CE

We standardised on the second solution because ListItem contains the position property which allows us to have ordered lists.

The CE's Json-ld output only goes "one level" deep when rendering an output. This means that we end up with something like this:

{"name": "our list", "itemListElement": [{"@id": "listitem-id"}, {"@id": "listitem-id2"}] }

in this case, the ids are of the ListItem. In order to get information about the actual things that these items refer to, an agent would have to first query the ItemList, then query each of the ListItems to get the ids of the actual targets, and then again make a query to each of these final target objects.

In order to minimise the number of queries that an agent has to do, we bring the information for each ListItem into the json for the ItemList, so we end up with something like:

{"name": "our list", "itemListElement": [
    {"@id": "listitem-id", "item": {"@id": "the thing's id"}}, 
    {"@id": "listitem-id2", "item": {"@id": "another thing's id"}}
]
}

[ChristiaanScheermeijer]

Nice!