Make FunkSVD use an updater object
Merge branch 'feature/elehack-funksvd-updater' of https://github.com/elehack/lenskit into feature/qiankun925-350-svd++
Fix bugs of svd++ (#350)
 - remove @notnull in the parameter list of SVDPlusPlusBuilder's constructor
 - change the update function name of item implicit features to itemImpUpdate
 - update the item implicit feature values for all items rated by the user
 - remove some wrong comments
 - decrease the iteration count to 50 in the accuracy test of SVDPlusPlusModel in lenskit-integration-test
Fix bugs of svd++ (#350)
 - remove @notnull in the parameter list of SVDPlusPlusBuilder's constructor
 - change the update function name of item implicit features to itemImpUpdate
 - update the item implicit feature values for all items rated by the user
 - remove some wrong comments
 - decrease the iteration count to 50 in the accuracy test of SVDPlusPlusModel in lenskit-integration-test