fix for serialization issue in kryo (stackoverflow error)

Both Spark and Scalding use Kryo (with reference tracking turned off) for serialization. I have been testing which breeze classes roundtrip Kryo serialization without issues. So far SparseVector, HashVector, DenseMatrix, and SliceVector all pass, but DenseVector causes a stackoverflow in Kryo. The issue with DenseVector is innerUpdate, which references back to variables in DenseVector and causes an infinite loop i guess in the serialization process. So i tried to modify it in such a way that it works with Kryo. 

Not sure if this is acceptable from a performance perspective.