Revise default value of solar absorption

Created by: mlauster

We currently set a default value of 0.7, I guess out of the validation case of VDI 6007-1. We always use this default value when relying on archetypes. According to http://www.ziegel.de/cgi-bin/tamzserver.exe/phpamz?id=505 this value relates to a red brick wall. The source defines also values for other materials. Maybe we should take a mean value as default value or set solar absorption in our materials.xml.