Add a single data point dataset to the tests
Created by: scarlehoff
This would fail with #1430 and adds two nice features:
- A single data point dataset is used in a variety of tests
- The group of data now contains two datasets from the same experiment which is another "corner case" which was not tested for.
The problem is that 2) breaks the tests for the covmat. So a few commits are needed to fix that.