 mapping of previous column:field selections

    This is useful when repeatedly importing from a third-party
    system: column names generated by the external system may
    not match Odoo's field names or labels. This model is used
    to save the mapping between column names and fields so that
    next time a user imports from the same third-party systems
    we can automatically match the columns to the correct field
    without them having to re-enter the mapping every single
    time.
    