[FIX] l10n_be: Fix BS and P&L

Multiple fixes in Belgian BS and P&L:
* Forgotten accounts
* Missing or empty sections
* Missing or wrong signs

Additionally, changes to the CoA in order to properly feed the BS and the P&L
sign=1 is the default value for account.financial.report, thus for a better clarity in the diff it would be good to remove unnecessary added lines such as this one