Clarify the expectations for when static data is included and make it configurable

[jleibs]

When static data is included in the result set it can be confusing.

The static data, when returned, might show up in row 0, but the row has no index values which can require extra user-side filtering.

Proposal

• Add a new enum along the lines of include_static to QueryExpression. The enum should have 3 values.
  • AUTO include a static row at row 0 only if there is static data in the dataset (the default)
  • ALWAYS row = 0 will be static regardless of whether there is static data
  • NEVER will never include an explicit row for the static data -- it will only be accessible via LatestAt.
• For AUTO / ALWAYS, the choice to include the static data is independent of any value of filtered_index_range.
• When using filtered_index_values or using_index_values :
  • The static data will only be included if the input includes TimeInt::STATIC
  • If ALWAYS and the input has no TimeInt::STATIC return an error
  • If NEVER and the input has a TimeInt::STATIC return an error.
• Note that we can implement a helper such as view(...).static_only() which is just an alias for:
  include_static=ALWAYS, filtered_index_values=[TmeInt::STATIC]

For 0.19

This does not need to be done for 0.19. Let's make the behavior match AUTO as default and add the configurabillity later.

• If filtered_index_values or using_index_values are set, then the choice to include static data is determined by an explicit TimeInt::STATIC in the input.
• Otherwise, if the view contains any column which includes static data, then row 0 will be the Static row, and this data will be returned regardless of the range specified by filtered_index_range
• We should clearly document: if there is static data in the results, it will always be in row 0 and that will be indicated by the the TimeInt for the index column being null

[jleibs]

Thinking about this more I'm now in favor of the alternative proposal:

• Always inline static data in every row of query results that selected a static column #7668

[teh-cmc]

• Closing in favor of Always inline static data in every row of query results that selected a static column #7668