Standardized definition of units in Modelica (also for money)

[otter]

Request from Francois Celler to introduce a standardized form how unit relationships are formally defined in Modelica. Especially, that also non SI units, such a "Dollar" etc. can be defined.
Should be discussed again in design group.

[HansOlsson]

Currencies are somewhat interesting for at least the following reasons:

• Time-varying conversion. Converting dollar to Euro we will need different conversions at different points in time. Thus if some cost is given in "Dollar" and someone wants to see the cost in "Euro" it will depend on the time.
  
• Time-varying - future values are often discounted in various ways to make them comparable. This discount of future values is model-dependent - and can be controversial - e.g. for evaluating the impact of global warning.
  

If the time is included in the unit the same variable will have different units over time; or a mapping from "time" in Modelica to actual time. If the time is not included in the unit the conversion to other currencies is not well-defined.

This will not always matter, but it will depend on the intended applications and thus more information is needed on the applications.