colour bar labels for generic plots do not show sufficient significant digits

[Joern Ungermann]

The cbar label format is determined according to the maximum value, which for logarithmic(-like) plots chooses too coars a format. In one example, "100, 0, 0, 0, 0, 0, 0, 0" is used.

[Reimar Bauer]

switched determining of cbar label format from maximal to median value

using the maximum value for logarithmic-like colorscales (autolog/nonlinear) prevented significant numbers from being shown. The median value should work much better for logarithmic and non-logarithmic colour scales on average.

Fixes issue #209

→ <<cset a947ae586fc4>>