Some code cleanup.

Use the "real" bit depth even for 16bit images by setting the sBit
attribute in the PNG format.

Problem: Bit depth is set in inherited class which is actually not known
here....
Partly closes #15.

This allows for getting rid of the "image type" attribute. The set
function is stubbed in this version.

Change to Atkinson algorithm for lambda>100. Closes #14.

Closes #106

scalingfactor in AnalyseFillfactor.