Upgrade to latest powerplantmatching

I had to pin powerplantmatching to 0.6.1. There are several reasons:

• Since 0.7 the MASTR data is directly available in ppm. Since we have our own workflow for the MASTR data (add_chps in add_existing_baseyear) this may loead to duplication of some German powerplants, particularly CHPs

• Since 0.7, and more so since 0.8, a lot of powerplants in DE get classified as CHP (probably to some interaction with the MASTR data). This happens because a lot of large PPs, particularly coal and lignite, have a minor waste heat utilization, which is recorded in MASTR, but which from the POV of pypsa-de does not justify classifying the whole PP as a CHP.

• Since they are now classified as CHP, the pypsa-de workflow, which takes all German CHPs directly from MASTR and hence drops all German CHPS contained in ppm, INCORRECTLY drops almost all coal and lignite units

• In ppm 0.8, something strange happend with the aggregation of several blocks, s.t. some capacities and efficiency are incorrectly represented, referring to historical values, e.g. for the Kraftwerk "Neurath"

Inspect the dependency pin and the add_existing_baseyear workflow, especially add_chps, to understand how powerplantmatching data is incorporated. Compare behavior with newer powerplantmatching releases, including CHP classification and the Neurath aggregation example. Done means the upgrade no longer duplicates German CHP plants or drops coal and lignite units, and capacities and efficiencies remain correct.