Add a citations() method to all cube APIs

[meggart]

When we want to convince data source providers to share their data in the datacube, they usually want to get their datasets properly cited. I think it would help to implement a method that outputs a list of reference citations for each dataset in the datacube, or the reference citations for a user-supplied subset of datasets. These reference citations would have to be exported by the dataset providers and stored somewhere in the cube datastructure.