Create DBSet

[ChrisBAshton]

Once #75 and #76 are done, it makes sense to remove the distinction between DBCreate and DBUpdate. Instead, have one DBSet object which handles both cases.

The application shouldn't need to know whether an object is existing and needs updating or whether that object is new and needs inserting in the database. It should just be able to call DBSet, which in turn would do the querying and then call DBCreate or DBUpdate (or some refactored equivalents).