Enhancement: support AbstractReaderClass subclasses as reader_class argument value

[robinmackaij]

Subclassing DataDriver when there is a need to use a custom reader_class is somewhat cumbersome at the moment since the init of DataDriver will try to resolve the reader_class based on filename. This can be improved by allowing AbstractReaderClass subclasses to be passed as the reader_class argument and having DataDriver check for subclass before trying to import the module from file path.