Now that we're accepting classifications, it's possible to specify account
options that select some but not all accounts at the same level of the
hierarchy. This commit tracks requested account names separately from sheet
names to do that correctly.
The period totals were reporting the balance of all the loaded postings, not
just the ones in the reporting date range.
Like the accrual report, introduce a RelatedPostings subclass that records
and saves all the information we need at group definition time, to help us
get it consistently right rather than redoing the same math over and over.
