Use more precise molecular weights

[chrisroadmap]

The motivation

I frequently convert between reporting units, e.g. Tg N, Tg NO and Tg NO2, Gt C and Gt CO2 when comparing datasets and running fair. I believe the conversions in OpenSCM units based on mass number are too imprecise; for example, Global Carbon Budget use 3.664 to convert C to CO2, which is 44.009 / 12.011; OpenSCM uses 44 / 12 = 3.667. I know the difference is well within emissions uncertainty, but affects harmonization and modelling runs (for instance fair uses PubChem weights).

openscm-units/src/openscm_units/_unit_registry.py

Lines 26 to 40 in 14a4e60

	"CO2": ["12/44 * C", "carbon_dioxide"],
	"CH4": "methane",
	"HC50": ["CH4"],
	"N2O": "nitrous_oxide",
	"N2ON": ["44/28 * N2O", "nitrous_oxide_farming_style"],
	"N": "nitrogen",
	"NO2": ["14/46 * N", "nitrogen_dioxide"],
	# aerosol precursors
	"NOx": "NOx",
	"NO": ["46/30 * NOx", "nox_cmip_style"],
	"nox": ["NOx"],
	"NH3": "NH3",
	"ammonia": ["NH3"],
	"S": "sulfur",
	"SO2": ["32/64 * S", "sulfur_dioxide"],

The proposed solution

Use PubChem or Wikipedia values for molecular weights, usually to three decimal places for these compounds.

Alternatives

Allow a user to override the default value.