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
As Karthik recently discovered in https://github.com/broadinstitute/gamgee/pull/375, the increment/decrement operators should return a reference to the iterator itself, rather than the value(s) it's pointing at (this is consistent with the way the STL works). Specifically, prefix increment should return a reference to the iterator after the increment, and postfix increment should return a reference to a copy of the iterator before the increment.

However, this is totally orthogonal to the change in this branch, so it's probably better to create a new ticket to address this (and to audit the increment/decrement operators across all of gamgee for consistency with the STL), instead of addressing it in this branch.