[IMP] l10n_be_hr_payroll: add Double Holidays/13 Month salary structures

addons/hr_payroll/models/hr_payslip.py

addons/hr_payroll/models/hr_salary_rule.py

addons/hr_payroll/views/hr_salary_rule_views.xml

...r_payroll/views/report_contributionregister_templates.xml

addons/hr_payroll/views/report_payslip_templates.xml

addons/hr_payroll/wizard/hr_payroll_payslips_by_employees.py

addons/hr_payroll_account/tests/test_hr_payroll_account.py

addons/l10n_be_hr_payroll/__manifest__.py

addons/l10n_be_hr_payroll/data/l10n_be_hr_payroll_data.xml

addons/l10n_be_hr_payroll/views/report_payslip_template.xml

addons/l10n_be_hr_payroll/views/reports.xml