Add non-greenhouse gas emission species

[danielhuppmann]

While working on some sanity-checks for the new IAMC project templates, I realized that several emissions species are relevant for IAM reporting but are not included in the list of emissions - because they are not greenhouse gases.

I suggest to add them anyway here so that pyam and a unit-validation-schema can directly build on ism-units (instead of requiring a separate solution).

I was trying to use the following line to check whether a unit like "Mt BC/yr" is valid.

iam_units.emissions.pattern.split(unit)

This fails for the following emissions species: BC, CO, NH3, NO2, OC, SO2, VOC, HFC43-10

Of course, trying to convert them using GWP should (continue to) fail.

Any objections @khaeru @phackstock?