Improve households data for Serbia

[mabijkerk]

Context

We have received a data suggestion from a user to improve the housing stock and typical heat demand data for Serbia.

Current shortcoming

Apparently the current data is not accurate and better information is available.

Proposed enhancement

The following data from this source is included:

Additionally, some data may be included in the Statistical Yearbook.

Success criteria

• We have tracked the current sources that are used for Serbia in the model
• We have decided how they compare to the two suggested sources listed above
• We have chosen if and how to update the data
• The data is updated and there are clear commit messages (these are now missing)