Invoices: added situation (or interim) invoices support

htdocs/accountancy/journal/sellsjournal.php

htdocs/compta/facture.php

htdocs/compta/facture/class/facture.class.php

htdocs/compta/facture/impayees.php

htdocs/compta/journal/sellsjournal.php

htdocs/compta/paiement.php

htdocs/compta/paiement/class/paiement.class.php

htdocs/compta/resultat/clientfourn.php

htdocs/compta/resultat/index.php

htdocs/compta/stats/cabyprodserv.php

htdocs/compta/stats/cabyuser.php

htdocs/compta/stats/casoc.php

htdocs/compta/stats/index.php

htdocs/core/class/commoninvoice.class.php

htdocs/core/class/commonobject.class.php

htdocs/core/class/html.form.class.php

htdocs/core/lib/pdf.lib.php

htdocs/core/lib/price.lib.php

htdocs/core/lib/tax.lib.php

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