Make temperature limit parameters actually optional

[jnettels]

This PR provides a fix that was, from my understanding, determined as fine in PR #76.

• The default temperatures are defined in the norm
• The temperature limits are defined two times in the script. In the other place, the defaults were already implemented. So this is only consequential

[p-snft]

As discussed in #76, this is following the implemented norm and thus fine.

PS: You might want to add this to the changelog.