Issues for evaluation of unseen user and incremental modeling(AddFeedback) in BPRMF

1. Passing null parameter to HashSet<int> when training_user_matrix[user_id] == null
===> ERROR: Argument cannot be null.
Parameter name: collection  at System.Collections.Generic.HashSet`1[System.Int32]..ctor (IEnumerable`1 collection, IEqualityComparer`1 comparer) [0x0006d] in /data1/daum/kskim/program/mono-2.10.8/mcs/class/System.Core/System.Collections.Generic/HashSet.cs:123
  at System.Collections.Generic.HashSet`1[System.Int32]..ctor (IEnumerable`1 collection) [0x00000] in <filename unknown>:0
  at MyMediaLite.Eval.Items+<Evaluate>c__AnonStoreyA.<>m__14 (Int32 user_id) [0x0002e] in /data1/daum/kskim/mymedialite.git/src/MyMediaLite/Eval/Items.cs:146
 
2. To perform incremental modeling in BPRMF after LoadModel, random instance need to be initialized