* price_average is now always positive regardless of the invoice type, because it's an average and there's no point in summing those values
* price_total is now computed by using ABS() because of a migration bug of price_subtotal_signed introduced in saas-6, where in_invoice and in_refund invoices were set with a negative value while on the help tooltip it was saying that in/out refunds are negative and in/out invoices are positive. That bug was only revealed in v9 -since saas-6 wasn't using that field at all-, with the effect of having wrong values in report for migrated databases. This workaround using ABS() will be removed in a further version with a migration script to fix people's databases.
* residual field is now negative for in_invoices and out_refunds.
* These 2 points ensure that we can group by invoice type and sum the price_total or the residual fields to get the gross income of a month for example.