CHANGE FEATURE: redesign dataset widget

Summary

The dataset content should rather be displayed in the DatasetViewer than a QTreeWidget

Reason

One cannot really see anything in the QTreeWidget using the default layout

Current behaviour

Each dataset is one topLevelItem in the tree with one child for the variables and one for the coordinates.

New behaviour

That should be turned into a table with two columns: one for the psyplot.data.DatasetAccessor.num and one for the file name (or None). The variables, etc. should than be displayed in the DatasetViewer (https://github.com/Chilipp/psyplot-gui/issues/3) if the user clicks on a row