Adjusting the layout of the components in the ColorGradientDialog
An attempt to make the code more readable

Creating a separate class for the node configuration dialog.
different syntax for the node settings objects.
Adding automatic configuration to the HeatMapViewerNodeModel (issue #1)

If the the node dialog is not opened, there is a new condition in the configure method of the HeatMapViewerNodeModel, that makes a default selection for the factors and the readouts if there are none given by the node settings.
Fixing a bug that was introduced two commits ago.

(happened in commit 112022c1d7a8acd8cc8d078fdb73cb22bf745435)
One setting name was used for two different settings, now the names are associated with the correct settings model again.
Bug fix for the error on closing the HeatMapViewerNodeView (issue #2)

I put a condition in the seralizeViewConfiguration method that checks if a binary file for the view configurations exists. If it exists, the node has been loaded already and the configuration has to be saved for the case that the workflow is not modified (bad hack). If the file does not exist, the routine is skipped when closing the view but will be taken care of in the saveInternals method of the node model, that will be called on closing knime, or the workflow, since there is a new node which sets the workflow modification flag.
It would be nicer to just set the workflow-is-modified (dirty) flag, so the view configuration has to be saved only once on closing the workflow and not each time on closing the view.
Adding functionality to ColorGradientDialog

the options panel to select the descriptor type and the factor to calculate the bounds is now inactivated when there are not reference populatons defined in the node configuration dialog.
Remodeling the HeatMapViewerNodeDialog.

There were some problems with the node settings when using the AbstractScreenTrafoDialog to create a column selector acting on a list selector containing the groups available in the nominal column. So I added code to create the custom dialog components in this class itself and modified the createControls method accordingly.
removed TODO list
cosmetics
Bug fix in the PlateViewer handling

Once a plate viewer was opened and then closed, it could not be re-opened again. The reason was that the window stayed invisible.