How many weapons violation crimes have occurred in the Calumet district?
"Calumet" is the district_name; 'WEAPON VIOLATION' is the primary_description of crime
SELECT SUM(CASE WHEN T3.district_name = 'Calumet' THEN 1 ELSE 0 END) FROM IUCR AS T1 INNER JOIN Crime AS T2 ON T2.iucr_no = T1.iucr_no INNER JOIN District AS T3 ON T3.district_no = T2.district_no WHERE T1.primary_description = 'WEAPONS VIOLATION'