Implementation of invoice tree logic to handle both creating view of existing items and merging with proposed item list.

There are a few changes introduced from the previous code:
- Full repair code has been deleted -- was only there for regression test purpose
- Pro-ration logic when calcultaing repair items gives a slightly different result-- and i think more accurate
- Handling of invoice item along with repair has also changed

Tree looks stable at this point, but more work (cleanup, refactoring and tests are needed)