Heat conduction bug fix: reference temperature

In the heat conduction process, a reference temperature was required. However, in the assembling that reference temperature was assigned to the temperature variable. This way, accessing the temperature always returned a constant reference temperature.

Now, the true temperature is assigned to the temperature variable and no reference temperature is required anymore. This way, temperature-dependent material properties can be implemented as well. On the other hand, if any material property explicitly requires a reference temperature, it still can be (and must be) provided using the medium property reference_temperature.

Deletions within and re-formatting of prj-files were made using xmlstarlet:

for FILE in *.prj; do 
	cp $FILE ${FILE}.orig
	# edit, i.e. remove tag 
	xmlstarlet ed -d  "/OpenGeoSysProject/media/medium[@id='0']/properties/property[name='reference_temperature']" $FILE.orig > $FILE.temp
	# reconstitute the OGS xml format
	xmlstarlet fo -s 4 $FILE.temp > $FILE
	rm ${FILE}.orig
	rm ${FILE}.temp
done

1. No new feature (description)
2. Tests for the heat conduction process already exist.