MVI and RBSVI now output pairs with file indices

- Made MVI() explicitly default
- added some consts and refs to tests

Enables use of the LUT (#376) by associating an input file index with every Variant
Due to the indeterminacy of the priority queue, we don't know which order they will come in.  But we do know which files we expect to contribute to each interval, so I'll add a checks for presence of the necessary files.