Add support for compounding indirect dues

[SudoVim]

Scenario:

PersonA owes PersonB $5.00
PersonB owes PersonC $7.00

We can change this to

PersonA owes PersonC $5.00
PersonB owes PersonC $2.00

This will also work if PersonA owes PersonB equal or less than what PersonB owes PersonC. We would have to obviously make multiple sweeps until we finally make no changes.

[SudoVim]

I'm not going to implement this until enough people want it.