Log colour scale of Generic Plotting Class inconsistent

[Joern Ungermann]

The vertical and horizontal generic plotting classes use manual computation of levels for log-plots depending on minimal and maximal values. The two class use inconsistent algorithms of varying stability against negativ or zero values.

This should be refactored using a common algorithm.

[Reimar Bauer]

Fixes issue #71.

→ <<cset a4a66ce0b1c7>>