Approximation of electricity distribution network losses

[nworbmot]

Insert a separation between the transmission network node and utility-scale assets, and the distribution level and distribution-level assets (like rooftop PV, BEV charging, home batteries).

[nworbmot]

The option to insert electricity distribution network capacities and associated expansion costs were added in https://github.com/PyPSA/pypsa-eur-sec/commit/9b326ba493c27191896e57bd6fdf930b3b354b20. Dealing with the losses is another matter, since I think they're already incorporated in the existing electricity load, so would have to be deducted and then added back.

[fneum]

closing in favour of #932