This allows period and comma to be used for decimal point and digit group
separator or vice versa, and also flexible digit groups. See
http://en.wikipedia.org/wiki/Decimal_separator . Digit group separators
are possibly not worth the trouble and might not stay.
Like ledger, price amounts are now "unobserved", ie their precision does
not affect the canonical display precisions used when displaying amounts,
and transaction balancing is done based on display precision, ie amounts
are considered to balance if their sum appears to be zero when using the
canonical display precision.
