Establish consistent information model/vocab

[khaeru]

The SDMX (Statistical Data and Metadata eXchange) ISO standard provides a thorough, consistent vocabulary, indeed an entire information model, for data and metadata: https://sdmx1.readthedocs.io/en/latest/resources.html

The item package should use these to better organize data that is processed. Specifically:

• Define data structures with specific dimensions for different measures. This avoids the need to have a large number of dimension columns, many of which are not relevant for certain measures. For instance, a “vehicle_type” dimension is never relevant for a “Population” measure. → done, #62
• Add the option to output the final historical database in SDMX formats.