Implement symmetrical classification methods for symbologies

[PhilippWinkemann]

Make it easier to visualize data using converging or diverging color schemes. Add a "symmetric classification" method to the dialog Classification (Layer Properties -> Symbology -> Quantities -> Graduated Symbols ->Classification)
This option allows us to create intervals / breaks around a given center. A classification using a center of 0.0, a width (bin size) of 2 and 5 classes will return the following classes:

> 3.0
1-0 --- 3.0
-1.0 --- 1.0
-3.0 --- 1.0
< -3.0

It would also be nice, to use increasing bin sizes.