dynamic line rating time series are 0 for lines under construction

[fneum]

There are ~300 lines which are under construction (under_construction = True and num_parallel = 0), which leads to unwanted zeros in the DLR time series here:

https://github.com/PyPSA/pypsa-eur/blob/1b88dc7660767099f49e58eaa778583a74defaba/scripts/build_line_rating.py#L126

Is there a good way to deal with this?

[fneum]

We should just avoid adding lines with num_parallel=0 ever as it causes many problems with power flow; see e.g. #444 #208.

Lines under construction should either be added with full capacity in the respective planning horizon when they are finished or if added as candidates, they should be links (i.e. no KVL association).