accounting_l10n_fr: Fixes for FEC

This change modifies the FEC export to conform to the test software from https://www.economie.gouv.fr/dgfip/outil-test-des-fichiers-des-ecritures-comptables-fec / https://github.com/DGFiP/Test-Compta-Demat.

A new test_compta_demat_compatibility to enable compatibility mode was introduced. This will be True when rendering the report from the dialog, i.e., using AccountingTransactionModule_viewFrenchAccountingTransactionFile and False when using AccountingTransactionModule_viewFrenchAccountingTransactionFileActive directly so the behavior of reusing FEC export for accounting interfaces should be same as before.