Closes # (if applicable).

Changes proposed in this Pull Request

• Bug fixes in add_existing_baseyear.py script
• Perfect foresight solving concatenates different pre-networks. Export to .nc requires datatypes for columns to be consistent. "reversed" column in n.links was previously mixed (True, False, nan) --> Set default for nan values to False
• Further, there was a missing space in the renaming of retrofitted gas boilers

Checklist

• I tested my contribution locally and it works as intended.