Add query methods

[alastair]

We need methods to be able to query items in the CE.
An advantage of GraphQL is that we can request exactly the fields and relationships that we need, and so I'm not sure how we should make these methods.
Some options:

• Provide a method to perform a request, but not to generate query text - let the user write the query that they want
• provide some generic query texts so that most uses are covered
• generate an interface that has parameters to include particular relations (this is the most complex solution and should only be done if we find a real need for it)

[pc2752]

Working on option 2.