WIP calculation and reporting improvements

Unbilled remainders now show correctly on aged WIP reports.
Debit memos are shown in all WIP views, so that the figures are consistent across all project screens.
Reversing credit memos now updates WIP appropriately.
WIP reversals can no longer be recorded for a date earlier than the WIP creation date.
Aged WIP reports now accurately reflect both billed and carried forward WIP. WIP is no longer overstated.