Make a clear distinction between derived quantities and basic inputs that are "backed out" from other inputs

[rplzzz]

There are a number of places in the GCAM data preparation where we start with some scenario assumptions about output and back out the basic quantities needed to cause the model to produce the desired output pathway. Here's an example: https://github.com/JGCRI/gcamdata/blob/70b1a2ffbfede3273b6037da0b0d13d573f85300/R/zchunk_L2321.cement.R#L376 This practice is problematic because if a later user tries to build a new scenario by, say, changing the projected GDP in one or more regions, then some of the effects of those changes will be sterilized by the backing out procedure.

What is happening here is that we are muddling what are inputs and what are outputs. The real input here is future cement production for the scenario, but we go through this procedure so that we can go through the motions of calculating these kinds of quantities as outputs. This creates a dangerous confusion about what is really being calculated and what kinds of related calculations are valid.

We need to be entirely clear on what quantities are inputs to our model, and what quantities are being calculated. Backed out quantities are inputs, and they should be presented to the data system as scenario parameters that we originated, not as derived quantities calculated from the other inputs.

To Do:

• [ ] Locate other instances of backing out in the data system.