A few motivations for this:
* This makes the fund report more maintainable, because the data structure
is better suited to the task at hand, making it easier to follow what's
going on.
* This helps put Balances through a little more testing with a high-level
report.
* This makes the fund report a little faster, since totals are usually
calculated from a smaller number of Balance objects rather than all
Postings over a period.
Make it look more like the spreadsheets:
* Don't normalize Expenses negative.
* Consistent account order: Income, then Expenses, then Equity.
* Include a bottom line divider for each fund.
This required keeping the balances from write_row, and then a lot of other
changes followed from that. In particular it makes more sense to build the
fund report sheet from scratch rather than copying the breakdowns report and
chiseling the fund report out of it.
