'Add a parameter to a field into script_fields
The ScriptFields object will be returned, so calls to this can be chained.'

def add_parameter(self, field_name, param_name, param_value):

    try:
        self.fields[field_name]['params'][param_name] = param_value
    except Exception as ex:
        raise ScriptFieldsError(('Error    adding    parameter    %s    with    value    %s    :%s' % (param_name, param_value, ex)))
    return self
