Non-transparent code: multiplying by 2

[robbieandrew]

CEDS/code/module-B/B1.2.add_SO2_comb_GAINS_control_percent.R

Line 291 in adf8394

### Why are we just multiplying by 2?

I presume the reason is that the data represent mass of sulphur and this multiplication converts to mass of SO2.
S = 32, O = 16, so one mole of SO2 is twice the mass of one mole of S.
This should be made more transparent in the code rather than just multiplying by 2.