Loading data with no DataReader node in experiment .yml file

Steps to reproduce:

1. Create a new experiment
2. Add image data files (raw, tiff, plain text)
3. Close OpenHKL
4. Create a new experiment with the same name as before
5. Add image data files

The error "No DataReader node in this yml file" is returned. The class ExperimentYAML saves all data reduction parameters but not the data reader parameters (i.e. the DataReader node).