Remove `{ll}` wildcard?

[koen-vg]

It looks to me like the {ll} wildcard remains mainly as a historical artefact -- I'm guessing this goes back to "The role of spatial scale in joint optimisations of generation and transmission for European highly renewable scenarios". However, today I don't really see any particular reason why transmission expansion limits should get their own wildcard? To me it would make perfect sense to make this a configuration option instead, and add a flag to the {opt}/{sector_opts} wildcards instead.

It's not really consequential, and the main drawback would be a disruptive change for users, having to deal with a new file-name structure.

If it's worth considering breaking backwards-compatibility on the file-name like this, there are some other things to consider:

• #708
• A little bit more radical, but I would love to merge the {opts}/{sector_opts} wildcards, eliminating confusion for new users and making it impossible to falsely put options in both of them. Instead, whether a network is electricity-only or sector-coupled should, in my opinion, be a configuration option.