Scadnano uses Modification.id as a unique identifier for modifications.

The web interface, for simplicity, hides this field from the user, and if they create a new modification, the field Modification.vendor_code is used also as the id.

However, any reasonable vendor code will already be unique to a particular type of modification. Thus it seems we don't really need the field id, and we can just use the vendor code everywhere as the unique identifier for the modification.

Codify this by removing the field id and just use vendor_code where id is currently used.