Build renewable profiles and add all assets after the clustering

[nworbmot]

• Currently renewable profiles and load are added to the high-resolution base network, then it is clustered. This makes the clustering highly dependent on the substation layout of the base network.
• This was an artefact of @coroa's original paper exploring clustering of the base network
• If we separate the clustering from the network, we are free to cluster on e.g. custom shapes, or cluster based on weather cells instead
• Generating renewable profiles after clustering requires fewer computations (num clusters instead of 5000 base substations)
• It would be easier to switch to allowing multiple VRE LCOE-classes inside each region rather than tying them to sub-regions.
• We would avoid the weirdnesses that some assets are added before, some added after the clustering - we could uniformise around PyPSA-Eur-Sec-style "assets added after clustering". This would simplify the workflow considerably.
• Downside: if you want to cluster based on VRE time series, you have to do extra work (but at least you're not tied to substation layout - you could e.g. cluster time series from the original weather cells)