 ``monetary`` converter, has a mandatory option
    ``display_currency`` only if field is not of type Monetary.
    Otherwise, if we are in presence of a monetary field, the field definition must
    have a currency_field attribute set.

    The currency is used for formatting *and rounding* of the float value. It
    is assumed that the linked res_currency has a non-empty rounding value and
    res.currency's ``round`` method is used to perform rounding.

    .. note:: the monetary converter internally adds the qweb context to its
              options mapping, so that the context is available to callees.
              It's set under the ``_values`` key.
    