def get_currency_conversion_session_for_type(self, source_currency_type=None, target_currency_type=None):
Gets an ``OsidSession`` associated with the currency conversion service and the given currency types.

        arg:    source_currency_type (osid.type.Type): the type of the
                source currency
        arg:    target_currency_type (osid.type.Type): the type of the
                target currency
        return: (osid.locale.CurrencyConversionSession) - a
                ``CurrencyConversionSession``
        raise:  NullArgument - ``source_currency_type`` or
                ``target_currency_type`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_currency_conversion()`` or
                ``supports_visible_federation()`` is ``false``
        raise:  Unsupported -
                ``supports_currency_types_for_conversion(source_currency
                _type, target_currency_type)`` is ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_currency_conversion()`` and
        ``supports_visible_federation()`` are ``true``.*