Option to create summary tables without adding "default" columns

[jperon]

Each time a summary table is created, default columns are added with sum of numerical columns. As a consequence, when creating many summary tables in which those sums don’t make sense, it’s a bit boring to have to delete those columns each time. It would be nice to have the option to create new summary tables with only the group column automatically added.

[vviers]

In an ideal workflow, being able to specify in the UI which summary functions to apply (or not!) to which columns when creating a summary table would be lovely :)

[jperon]

Sure. And due to #361, getting a warning when defining a summary function whose name is already in use in another summary of the same table would be another great point.

[dsagal]

This project https://github.com/gristlabs/grist-core/issues/705 (soon to be released) will actually make it much easier to add the kinds of columns that summary tables today create automatically, as well as to select other common functions.

This feature doesn't yet help with summary tables, but it's makes it easy to imagine a follow-up where we could make summary tables start out with no columns, but with an easy way to add any needed columns.

[jperon]

While "restoring" summary tables whose group columns I had deleted, I saw two points about this issue:

• the only way to create a group column is to have it automatically created by creating a view with the exact same "group by" fields: trying to manually add a group column with table.getSummarySourceGroup(rec) doesn’t work;
• the automatically addition of "default" columns this issue intends to avoid is specially annoying when having to do such a repair on multiple summary tables: not only it adds sums, but it adds formulas from other summary tables that don’t fit with the new ones (and which frequently raise exceptions). This point is related to #361 too.