Other kinds of parameters

Right now, we have three kinds of parameters: Quantities, operators, and reducers.

For some indices, we need parameters such as reference periods or maybe others. To allow for the passing of these, different options are possible:

• Introduce new kinds as needed (increasing code complexity, but making the index specification type-safer)
• Encode everything in the current kinds (reference period would be a quantity)
• Introduce some form of lightweight kind, say kind: scalar that supports simple data with no units. For this case reference period would probably be a string following the cmip6 convention for time_range (see page 14f)

@a001257, what do you think?