``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.