Computes how a user vector Xu changes in response to interaction with an item vector Yi.
This can also be used to compute how an item vector changes in response to a user interaction,
even though the code naming follows the former convention.
solver - solver helping solve for Xu in Qu*Y = Xu * (Yt * Y)
value - strength of interaction
Xu - current user vector (null if no existing user vector)
Yi - current item vector
implicit - whether the model is implicit feedback
new user vector Xu, or null if no update should be made (i.e. there was no
item vector; the update would push the new Qui farther out of range)