Add SDMX input/output

[khaeru]

This would add to .compat.sdmx operators like…

• Convert sdmx.model.DataSet into Quantity.
• Perform a specific SDMX query to retrieve data.
• Convert Quantity into sdmx.model.DataSet.

Some issues to resolve here:

1. Quantity.attrs map well to SDMX attributes attached at the level of an entire data set. However, one powerful feature of SDMX is the ability to attach attributes to individual observations. This does not have a natural analogue in the xarray (thus genno) data model.

[khaeru]

1. Quantity.attrs map well to SDMX attributes attached at the level of an entire data set. However, one powerful feature of SDMX is the ability to attach attributes to individual observations. This does not have a natural analogue in the xarray (thus genno) data model.

One initial way to resolve this would be to provide a feature to "merge" ≥2 Quantities together: one providing the observation values and others, of the same shape, providing attribute values.

Another way to support this would be to provide a generally "labeller" ("attributor") callback that attaches attributes to each observation.

In both of these designs, it is still up to the user to track and modify attributes through the graph of computations, unlike the automatic way units are handled.