Group payments for easier summaries

[whiterook6]

It looks so far like payments will come in groups.

• For X months, you get paid Y dollars
• Then, something changes, and your monthly payment changes to Z
• etc.

Given a set of rules, instead of generating a list of months (or in addition to generating a list of months),

• generate a set of groups, each with a start, an end, and some stats about the middles.

For example, you might show the first two payments, then omit 56 of them, then show the final 2 payments of a group of 60 payments. You would also show the payment count, the total earned, split up, etc.

Ideally a paymentGroup should be able to generate the missing months.