Add windowLength to SelectorDefault

The default selector can now include all fields within a certain time
window. This acts like a time-smoothing mechanism.
Improves graphics products. Adds spreadSkill verification.

Improvements to graphics
* Change the x-axis to UTC for CdfPlots.
* Improve the grid lines
* Add spread-skill plot
* Allow specification of offset
	modified:   graphics/meteo/Plot.py
	modified:   graphics/meteo/File.py
	modified:   graphics/meteo/verif.py