Define attributes needed for data-driven groupings

[ASL-rmarshall]

A data-driven grouping is a grouping where the values to be included in the analysis are not prespecified as part of the definition of the analysis. Things to be clarified:

• If you include a specification of groups as part of the design of the analysis, do you want to be able to indicate whether:
  • All the defined groups are displayed (with zero values for groups that did not occur).
  • Only groups that appear in the data are displayed?
• Do you need to be able to specify a "display format" for data-driven group values (e.g. if this data value occurs, this is how it should be displayed)? If so, is this any different to prespecified groups?
• How should dummy values for data-driven groups (needed for creation of the display shell) be defined?

[drewcdisc]

These questions need to be move to Discussions and then we'll add PBIs to the Backlog once we've answered the questions.

[drewcdisc]

@ASL-rmarshall @bhavinbusa I converted Issue #67 into Discussion #142. After all the questions in here are answered we can create independent PBIs that may arise from the answers.