Generate runtime warnings for naive implementations

There are some example code under src/main/scala/org/apache/spark/examples:

• LocalALS
• LocalFileLR
• LocalKMeans
• LocalLP
• SparkALS
• SparkHdfsLR
• SparkKMeans
• SparkLR

They provide naive implementations of some machine learning algorithms that are already covered in MLlib. It is okay to keep them because the implementation is straightforward and easy to read. However, we should generate warning messages at runtime and point users to MLlib's implementation, in case users use them in practice.