We have many `DataType`s backed by the same `DataStore`. For example,
observations and tracks are both in the "data" store.

`DataType.prototype.getByVersionId` would pull documents from the store
without checking the type, which could cause issues. Simple code
example:

    const track = await trackDataType.create({ /* ... */ })

    await observationDataType.getByVersionId(track.versionId)
    // => the track, unexpectedly!

Now, it throws a `NotFoundError`.