add contexual field to invoice form and copy value from sales if available

sale_order_contextual/models/account_move.py

@@ -0,0 +1,9 @@
+from odoo import fields, models
+
+
+class AccountMoveContextual(models.Model):
+    _inherit = 'account.move'
+
+    contextual_note = fields.Text(
+        string='Contextuele opmerking',
+    )