Standard unit support for all emissions species

[gidden]

In reviewing a PR for aneris I noticed that the following passes

df.convert_unit('Mt CO2/yr', to='kt CO2/yr')

and the following fails

df.convert_unit('Mt BC/yr', to='kt BC/yr')

It's not urgent, but I'd be keen for pyam to support at least simple unit conversions for all emissions species IAMs use. If someone (@danielhuppmann?) can point me in the right direction to get started, happy to tackle this (eventually)

[danielhuppmann]

Black carbon isn't listed in https://github.com/openclimatedata/globalwarmingpotentials and the derived emissions species in https://github.com/IAMconsortium/units, not sure why...

Options for resolving this:

1. Add BC to those packages (no need to make any changes in pyam, just update iam-units)
2. If there is a good reason why BC shouldn't be treated in those packages, make an alternative proposal