Added the hasGravity method to Blocks. Adds BUKKIT-3832

By: nitnelave <nitnelave1@gmail.com>

paper-api/src/main/java/org/bukkit/Material.java

@@ -952,4 +952,21 @@ public enum Material {
                 return false;
         }
     }
+
+    /**
+     * @return True if this material is affected by gravity.
+     */
+    public boolean hasGravity() {
+        if (!isBlock()) {
+            return false;
+        }
+        switch (this) {
+            case SAND:
+            case GRAVEL:
+            case ANVIL:
+                return true;
+            default:
+                return false;
+        }
+    }
 }