Make BasicLoader::valueAt() non pure virtual.

Also in base implementation `assert(false)`, so that it is not used.
This protect us at least when -ea is provided to VM.

src/main/java/azgracompress/io/loader/BasicLoader.java

@@ -22,7 +22,10 @@ public abstract class BasicLoader {
      */
     public abstract int[] loadPlaneData(final int plane) throws IOException;
 
-    protected abstract int valueAt(final int plane, final int offset);
+    protected int valueAt(final int plane, final int offset) {
+        assert (false) : "Unimplemented overload of BasicLoader::valueAt()";
+        return Integer.MIN_VALUE;
+    }
 
 
     /**

src/main/java/azgracompress/io/loader/RawDataLoader.java

@@ -47,12 +47,6 @@ public final class RawDataLoader extends BasicLoader implements IPlaneLoader {
         return TypeConverter.unsignedShortBytesToIntArray(buffer);
     }
 
-    @Override
-    protected int valueAt(int plane, int offset) {
-        // TODO: Implement me!
-        return 0;
-    }
-
     @Override
     public int[] loadPlanesU16Data(int[] planes) throws IOException {
         if (planes.length < 1) {

src/main/java/azgracompress/io/loader/SCIFIOLoader.java

@@ -40,12 +40,6 @@ public final class SCIFIOLoader extends BasicLoader implements IPlaneLoader {
         return TypeConverter.unsignedShortBytesToIntArray(planeBytes);
     }
 
-    @Override
-    protected int valueAt(int plane, int offset) {
-        // TODO: Implement me!
-        return 0;
-    }
-
     @Override
     public int[] loadPlanesU16Data(int[] planes) throws IOException {
         if (planes.length < 1) {