[FIX] account: error in Base amount tax

opw-1917605

Before this commit, when accounting is set to global rounding, the base
amount per tax is not correctly calculated.

Now, the Base amount per tax is calculated adding the rounded lines.

To test this issue, you need to set to global rounding and create a
customer invoice with the following lines :
line 1 = Qty:40; unit price:2.27; tax:15%; discount: 10%
line 2 = Qty:21; unit price:2.77; tax:15%; discount: 10%
line 3 = Qty:21; unit price:2.77; tax:15%; discount: 10%

When the invoce is printed, you would notice that the amount is 186.43
and it should be 186.42.

closes odoo/odoo#30021