Review model sections that set values relative to start year

[kndehaan]

Various sections in the model set values relative to the start year. This doesn't work if in the start year, the specific (sub)sector does not exist. It is also not possible for users to set future demand of these sub sectors that do not exist in the start year. Specific examples:

• relative size slider in industry sectors to set the relative increase or decrease of the sub sector
• sliders in agriculture that set relative increase or decrease in electricity and heat demand

We should review the different model sections where this issue might persist. A solution could be to use absolute sliders that set the size of the industry (similar to what is done for fossil refinery).