Multi-column balance reports, used by the balance command.
A multi balance report is a balance report with one or more columns. It has:
- a list of each column's date span
- a list of rows, each containing a renderable account name and the amounts to show in each column
- a list of each column's final total
A row in a multi balance report has
- An account name, with rendering hints
- A list of amounts to be shown in each of the report's columns.