Strict vs. permissive handling of missing/dimensionless units

[khaeru]

Consider these cases:

>>> from genno import Quantity, computations
# Case A
>>> computations.add(Quantity(1.0, units="kg"), Quantity(2.0, units="tonne), Quantity(3.0))
ValueError: Units 'kg' and '' are incompatible
# Case B
>>> computations.add(Quantity(1.0, units="kg"), Quantity(2.0, units="tonne"), Quantity(3.0, units=""))
ValueError: Units 'kg' and '' are incompatible

In (A) collect_units() assigns dimensionless to the last operand. In (B), it is explicitly dimensionless. This arose in iiasa/message_ix#441, where computations.add() is applied to two quantities, one with units, the other dimensionless (because the ixmp parameter handled by ixmp.reporting.computations.data_for_quantity() was empty).

What should the behaviour be?

Some possibilities:

• In (A), infer that operand(s) with missing units is in the same units as the first/others. Maybe only if the units are consistent?
• In (B), infer that explicitly dimensionless operand(s) are in the same units as the first/others.
• Add a (global?) configuration setting to toggle between different behaviours. (What should be the default?)