Consider allowing reuse of vector object in View.getVector

[jkni]

The View interface returns a unique object for every call to getVector, but the object returned by getNeighborsIterator is only guaranteed to be usable until the next call to getNeighborsIterator. If we put getVector under the same constract, we may be able to optimize the implementation for some readers and reduce object allocation.

[jbellis]

(low priority imo b/c the reranker only calls getVectorInto and not getVector)