This is the interface for handling CalFile records as they are read by a VariableConverter.
The handler method accepts a CalFile* as the sole argument, and it should return true if it handled the record, or false if the record should be handled by VariableConverter::parseFields(), the default record handler. When the handler is called, the current fields and time from the CalFile record can be retrieved with CalFile::getCurrentFields() and CalFile::getCurrentTime().
See makeCalFileHandler() and the CalFileHandlerFunction template for a simple way to create implementations of this interface which call existing functions, such as class methods.