This upgrades Account to enable it to store a multiperiod balance, with
a separate balance for each date period. This enables it do the hard
work in MultiBalanceReport.
Some new types are created to enable convenient operation of accounts.
- `BalanceData` is a type which stores an exclusive balance, inclusive
balance, and number of postings. This was previously directly stored
in Account, but is now factored into a separate data type.
- `PeriodData` is a container which stores date-indexed data, as well as
pre-period data. In post cases, this represents the report spans,
along with the historical data.
- Account becomes polymorphic, allowing customisation of the type of
data it stores. This will usually be `BalanceData`, but in
`BudgetReport` it can use `These BalanceData BalanceData` to store
both actuals and budgets in the same structure. The data structure
changes to contain a `PeriodData`, allowing multiperiod accounts.
Some minor changes are made to behaviour for consistency:
- --declared treats parent accounts consistently.
- --flat --empty ensures that implied accounts with no postings are not displayed, but
accounts with zero balance and actual postings are.
