CHP plants remain as existing capacities despite expired lifetime

Running the myopic Code, CHP plants which are supposed to be filtered out in add_existing_baseyear due to their expired lifetime remain within the network. The filter does not work, as several data points have missing values in the columns queried in https://github.com/PyPSA/pypsa-de/blob/ca2d8ff3502115034a0adca5ad8ed0aef2bae973/scripts/add_existing_baseyear.py#L463

A potential solution could be to provide a default value for the plants' lifetimes where it is missing and to filter out plants whose lifetimes have expired:

    chp["lifetime"] = (chp.DateOut - chp["grouping_year"] + 1).fillna(
        snakemake.params.costs["fill_values"]["lifetime"]
    )
    chp = chp.loc[chp.grouping_year + chp.lifetime >= baseyear]