Add UK statistics to the repo & update data folder structure

[sophieoneill]

Add UK based statistics for population, diurnal patterns, water use etc to the repo. To do this we are proposing to update the structure of the pysimdeum/data folder. Rather than adding a second UK folder replicating the NL one, which would result in lots of duplicate .toml files, we would:

• remove the NL folder, so all files are moved to directly under data
• Update each file so that it has two layers, NL and UK, with related data under each

The default will be set to NL but with an optional flag that allows the users to use the UK data instead.
@BramHillebrand Does this sound ok?

[sophieoneill]

After discussions we have decided to add the UK data to the repo by replicating the NL folder structure, not the methodology describe in this issue. This is being done in PR #73