Identify codes that respresent totals

[Robsteranium]

For the data previews (#83), it'd be nice to identify those codes which will give the margin of the distribution for that dimension - i.e. typically the "total" or "average" - to that we can pick defaults for unspecified dimensions and leads fact finders more quickly to single observation values.

This will require us to decide on a model and load the necessary data.