Supported vector operators in scala should have parity with pySpark

pySpark supports various overloaded operators for the DenseVector type that the scala class does not support.

• ML: https://github.com/apache/spark/blob/master/python/pyspark/ml/linalg/__init__.py#L441-L462
• MLLIB: https://github.com/apache/spark/blob/master/python/pyspark/mllib/linalg/__init__.py#L485-L506

We should be able to leverage the BLAS wrappers to implement these methods on the scala side.