Fix update issue.

Preliminary fix issue in previous commit.

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