please show some warning when attempting to group by non-searchable custom field
Per issue #54, when a non-searchable custom field has the Aggregate box checked, the expected behavior does not result.
This behavior is implemented in https://lab.civicrm.org/extensions/dataprocessor/-/blob/master/Civi/DataProcessor/DataSpecification/Utils.php#L56
Since the user has indicated, by checking the Aggregate box, that they wish for this behavior to happen, there should at minimum be a warning that it will not happen, and directing them to make the field searchable to fix this.