FIX VAT rate can be negative. Example spain selling to morroco.

.../core/modules/askpricesupplier/doc/pdf_aurore.modules.php

htdocs/core/modules/commande/doc/pdf_einstein.modules.php

htdocs/core/modules/commande/doc/pdf_proforma.modules.php

htdocs/core/modules/facture/doc/pdf_crabe.modules.php

htdocs/core/modules/propale/doc/pdf_azur.modules.php