add dyn cast to sbp

sbp/src/operators/upwind4.rs

@@ -413,6 +413,10 @@ impl UpwindOperator1d for Upwind4 {
             fut,
         )
     }
+
+    fn as_sbp(&self) -> &dyn SbpOperator1d {
+        self
+    }
 }
 
 impl<SBP: UpwindOperator1d> UpwindOperator2d for (&Upwind4, &SBP) {

sbp/src/operators/upwind4h2.rs

@@ -92,4 +92,8 @@ impl UpwindOperator1d for Upwind4h2 {
             fut,
         )
     }
+
+    fn as_sbp(&self) -> &dyn SbpOperator1d {
+        self
+    }
 }

sbp/src/operators/upwind9.rs

@@ -72,6 +72,10 @@ impl UpwindOperator1d for Upwind9 {
             fut,
         )
     }
+
+    fn as_sbp(&self) -> &dyn SbpOperator1d {
+        self
+    }
 }
 
 #[test]

sbp/src/operators/upwind9h2.rs

@@ -100,4 +100,7 @@ impl UpwindOperator1d for Upwind9h2 {
             fut,
         )
     }
+    fn as_sbp(&self) -> &dyn SbpOperator1d {
+        self
+    }
 }