Add European load profiles

[brynpickering]

Closes #70

A few things to note:

1. heat demand isn't included yet, so the distribution of total Eur demand will be wrong (since it is dividing it only into baseline electricity and transport)
2. the process of weighting the different demands is a bit of a mind melt. What I do is say (e.g.):

• From historical data, GB in 2012 has a 0.5:1 base:transport demand ratio, NO has a 1.67:1 ratio
• GB in 2042 has a 1.41:1 ratio, so NO must have a 1.67*(0.5/1.41):1 ratio = 4.55:1
• this assumes the relative importance of the two sources of demand, when compared between NO and GB, stay the same through time

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in config/config.GB.yaml.
• Changes in configuration options are documented in doc/gb-model/configtables/*.csv.
• OET SPDX license header added to all touched files.
• Sources of newly added data are documented in doc/gb-model/data_sources.rst.
• A release note doc/gb-model/release_notes.rst is added.