accounting: round in grouping when no section currency is set

Grouping feature checks that the sum of all selected lines == 0, which
is often not the case as the values are float. For that, our approach
is to round the values with the precision of the accounting currency,
since these precisions are usually small (typically 0, 2 or 3), we
don't have problems with rounding. Using the section currency is not
just a workaround for rounding, it's also correct because we don't
consider more precise amounts in accounting transaction lines.

The problem with this approach was for the case where no accounting
currency is set on the section organisation, in that case we did not
round and this sometimes led to "grouping is impossible" errors that
are hard to find for users. At this level it's better to use a default
rounding precision that would make it possible to use the grouping
feature even when section currency is not set.