Add stubbed-out block processing to fork choice

eth2/lmd_ghost/src/reduced_tree.rs

@@ -40,7 +40,7 @@ where
         }
     }
 
-    fn process_message(
+    fn process_attestation(
         &self,
         validator_index: usize,
         block_hash: Hash256,
@@ -52,6 +52,11 @@ where
             .map_err(Into::into)
     }
 
+    /// Process a block that was seen on the network.
+    fn process_block(&self, block_hash: Hash256, block_slot: Slot) -> SuperResult<()> {
+        unimplemented!();
+    }
+
     fn find_head<F>(&self, start_block_root: Hash256, weight_fn: F) -> SuperResult<Hash256>
     where
         F: Fn(usize) -> Option<u64> + Copy,