FIX: allow creating SS_Reports with sourceQuery, instead of only sourceRecords

The code suggests that you can implement either `sourceQuery` or `sourceRecords`:
Report.php line 112: "Please override sourceQuery()/sourceRecords()"

This PR fixes things so that you can in fact do that. Does not break backwards compatibility.
Do you think asserting types is enough? (assertInstanceOf)
I'm thinking there is more the report test could cover. Most of it seems to be about including or excluding reports.