Simplify normalizing I-I model builder configuration

- Add vector truncator provider to simplify reproducing standard behavior (this will be even simpler after fixing grouplens/grapht#81)
- Improve normalizing item-item docs
- Make normalizing item-item integration test use new truncator
Make normalizing builder take advantage of sparsity
Add test for item-item variants producing the same recs
Add time logging to normalizing builder
Use iteration strategy for sparisty in normalizing I-I builder
Write prediction channels in item-item test output

- Fix prediction output metric to write channels
- Write neighborhood size channel in item-item method test
Handle unpredictable items in item-item verification
Drop unneeded Python script
Clean up imports
Merge pull request #583 from elehack/feature/elehack-simple-normalizing

Improve and bug-fix normalizing item-item builder