Build some kind of mental model for external data

* "External itemsets": https://opendatakit.org/help/form-design/external-itemsets/ (http://xlsform.org/#external-selects)
- Very narrow. Allows for building a CSV in the same structure as the choices sheet that gets imported to a database to speed up large lists.

* "Pulldata": https://opendatakit.org/help/form-design/data-preloading/ (http://xlsform.org/#pre-loading-csv-data)
- Function that can be used in calculations to pull a cell from a CSV.

* "Search appearance-function": http://xlsform.org/#dynamic-selects-from-pre-loaded-data
- Function used in appearance that pulls a column from a CSV to use as a choice list.

These have overlap in functionality.

The XForms spec-compliant way to do all this and be able to use XPath for querying is https://opendatakit.github.io/xforms-spec/#secondary-instances---external.

Let's punt on this until we have a rough plan of how to resolve some of these historical challenges.

Start by reading the linked documentation on external itemsets, pulldata, search appearances, and XForms secondary instances. Compare the overlapping approaches and record a rough plan for resolving the historical challenges; done means the project has an agreed mental model and direction.

Written by the indexing model from the issue text.