Default columnSizing of `group-by-<field>` columns to `<field>` values

[roblotter]

If columnSizing does not have an entry for group-by-<field> cols, take the value from <field> if one exists.

Maybe see if we can do this for other properties, not only columnSizing

From initial proposal in #11 :

Also, it would be great if, when we have groups and group-by- is not defined in columnSizing but is, then the group size could be set by default to the value given for the . This will allow to have only one definition (no matter if it's a group or not) and would be more convenient when the configuration of the groups are dynamic (as a user input for instance).