Sub-queries in select clause
@param query
@returns {IQF}
function field(query) {
    var queryDefinition;
    if (query instanceof Function) {
        queryDefinition = query();
    }
    else {
        queryDefinition = query;
    }
    var customField = queryDefinition.select;
    customField = customField.addSubQuery(queryDefinition);
    // Field query cannot be joined to any other query so don't have set the positional fields
    return customField;
}