Add method for get section.

Author: ivanvorobei

Example Apps/SPDiffable.xcodeproj/project.xcworkspace/xcuserdata/ivanvorobei.xcuserdatad/UserInterfaceState.xcuserstate

@@ -147,11 +147,18 @@ open class SPDiffableCollectionDataSource: UICollectionViewDiffableDataSource<SP
         return itemIdentifier(for: indexPath)
     }
 
+    /**
+     SPDiffable: Get sections.
+     */
+    public func sections() -> [SPDiffableSection] {
+        return snapshot().sectionIdentifiers
+    }
+    
     /**
      SPDiffable: Get section by index.
      */
     public func section(for index: Int) -> SPDiffableSection? {
-        let snapshot = self.snapshot()
+        let snapshot = snapshot()
         guard index < snapshot.sectionIdentifiers.count else { return nil }
         return snapshot.sectionIdentifiers[index]
     }