Requirements gathering for category conversions

[mikapfl]

• we should have a way to introduce (ad-hoc?) summed-up categories for cases where there are unresolved sums on the target categorization -> extended cateogirzation, extend conversion with new rules to new categorization
• maybe easier to specify one-way conversions instead of symmetric ones
• maybe method to store extended categorizations outside climate_categories, e.g. country-specific categories.
• conversions need to be extended with new rules, or existing rules need to be removed
• we need the possibility to use nansum
• we do not want downscaling to happen during the conversion
• format for conversions should be extensible to multidimensional rules (maybe just add row with "lhs", "aux", "rhs" so that it is clear if dimensions are for the input, output, or just filtering)