accounting: fix validation of accounting transactions with too precise amounts

Round the amounts line by line when validating transactions balances and
showing the totals on an accounting transaction.

Even though float field configurations do not allow users to input transactions
where amounts uses more digits that what makes sense for the selected currency,
we had cases where some transactions generated by custom scripts were "too
precise" and passed the validation because unlike in all reports where we
display sum of rounded values, the check was only rounding the sum, which cause
some transactions with 1.00 on one side and 0.333333 0.333333 and 0.333333 on
the other side to be valid.

The scripts used to display sums on an accounting transaction had the same
problem, they were also showing the rounded sum and not the sum of rounded
values.