First try using the BufferedDataTableHolder to access the image data

There are a lot of modifications, just to quickly see if I can get it running. So far no bigger performence improvements could be observed, however I did not test it with a huge table. The PlateViewer still renders slowly though. I also have serious doubts if the image table access is not too slow, iterating each time through the table until it finds the corresponding line.
fixed Colormap problem (issue #15)

The bounds of the color gradient were calculated wrongly, instead of getting the closest two colors to the value we want to interpolate, the upper bound was always the last one.
Adding another default color map (red-white-blue)
Cleanup of the modifications for the BufferedTableHolder changes

Move the implementation of the BufferedDataTableHolder interface from the HeatMapViewerNodeModel to the HeadMapModel.
Removing redundant code.
Adding doc-comments
Adding menu-icon for the new red-white-blue colormap
Merge branch 'img-table' into kos
Adding general Node stuff

Better handling of empty tables.
Adding checks for workflow execution cancellation.
Adding progress bar update
Global color scaling bugfix

if there was no Plateviewer open the state of the menu-checkbox for the global color scale was not updated in the HeatMapModel, thus the a new (first) PlateViewer would not be initialized correctly.
cleaning the code

introducing a private method for better reading.
adjusting test method

since the HeatMapViewerNodeModel has some new method signatures (commit 8a54c046517f8543e5653775ee370032e986c018), the testing class needed to copy the method since there is no ExecutionContext when running the "naked" GUI without KNIME.
Removing the fields of the Well class that were holding image data.

The Well class was initially intended to hold the image data. Now by making the HeatMapModel a BufferedTableHolder, this is not necessairy anymore.
Using a double buffered approach to render the PlateViewer (issue #4)

The PlateViewer got a new draw method for initially drawing the UI, afterwards the updating is made with the repaint().
The Graphics object created by the JFrame (BufferStrategy) needs to be passed to the HeatPlate and the HeatWell. The signatures were adapted accordingly.
adjusting test method

The new renddering approach (previous commit) needed adjustments...
Removing unnecessary statements
Adding an image output port to the HeatMapViewer (feature request #19)

The configure and execute routines of the HeatMapViewerNodeModel and an additional method has been added that creates the image output.
The HeatTrellis and HeatMapColorToolBar needed some adjustmets to makes certain properties available from outside. For instance the font need to be bigger when rendering it for an image.
Finally the AbstractNodeModel needed a new constructor to be able to hold the new port configuration of the HeatMapViewerNodeModel