public class InLinkBlock
implements scala.Product, scala.Serializable
In-link information for a user (or product) block. This includes the original user/product IDs
of the elements within this block, as well as an array of indices and ratings that specify
which user in the block will be rated by which products from each product block (or vice-versa).
Specifically, if this InLinkBlock is for users, ratingsForBlock(b)(i) will contain two arrays,
indices and ratings, for the i'th product that will be sent to us by product block b (call this
P). These arrays represent the users that product P had ratings for (by their index in this
block), as well as the corresponding rating for each one. We can thus use this information when
we get product block b's message to update the corresponding users.