New country datasets NO, CH, RS, GB

[kndehaan]

With this PR, the new country datasets for Norway, Switzerland and Republic of Serbia are added. This project contains the following additions / changes to the ETM:

• New country datasets NO, CH, RS, including energy balance data and other data keys
• Source analysis files for deriving the origin of data used to make the datasets
• New country dataset GB, created from the UK and Northern Island dataset using the new reversed amalgamator tool
• Curves for the new country datasets
• Improvements of various tools and scripts, e.g. the Energy Balance Generator

Goes with:

• https://github.com/quintel/etlocal/pull/553
• https://github.com/quintel/etdataset/pull/1020
• https://github.com/quintel/etmodel/pull/4354
• https://github.com/quintel/etsource/pull/3132

[mabijkerk]

Very minor detail, but the new flags look different in terms of style:

Also, while we're at it, can we add a flag for Singapore?

For some reason the flag doesn't load for "Continue" "Your current scenario...":