Update unit_conversions.jl with comment from #86

src/Utils/unit_conversions.jl

@@ -1,7 +1,8 @@
 const year = 365.25 * 24. * 60. * 60.
 
+#  Multiplying by 2.13 converts from parts per million CO2 to Gigatonnes of carbon (https://web.archive.org/web/20170118004650/http://cdiac.ornl.gov/pns/convert.html) and then multiplying by `44/12` gets us back to mass of CO2 (12u + 2*16u = 44u compared to 12u for just elementary carbon). 
 GtCO2_to_ppm(GtCO2) = GtCO2 / (2.13 * (44. /12.))
 tCO2_to_ppm(tCO2) = GtCO2_to_ppm(tCO2) * 1.e-9
 
 ppm_to_GtCO2(ppm) = ppm * (2.13 * (44. /12.))
-ppm_to_tCO2(ppm) = ppm_to_GtCO2(ppm) * 1.e9
+ppm_to_tCO2(ppm) = ppm_to_GtCO2(ppm) * 1.e9