- When None values are observed, ROC and Enrichment were computed
  inconsistently since the order in which the values occured played a
  role. Now, None values are ignored.
- Enrichment was computed inconsistently, when the same value occured
  multiple times. Now, the average enrichment is taken, similar as in
  ROC calculation.

when adding a new column

in case of an empty table (what we actually want...)

can be outsourced in the corresponding insert chain function

relative paths

Setting the PYTHONPATH in ost_config is no longer required (This was
breaking stuff for projects relying on OpenStructure).

It's causing problem on some systems

This allows to set the title to an empty string

The scene background can now be a gradient or a bitmap image in addition to a static color; use scene.SetBackground(gfx.Gradient) or scene.SetBackground(gfx.Bitmap) for this purpose.

The gfx.Bitmap class and its import/export functions have been exported to Python.

Some cleanup in the OpenGL code to (i) avoid premature rendering before full initialization and (ii) streamline error checking a bit.