Unify decorated vectors in DecoratedVector<T>

I'm finding the current Vector classes in Mahout a bit confusing.
The vector implementation are just fine, I'm talking more about the decorated vectors:
WeightedVector
MatrixSlice
NamedVector

I propose using a single DecoratedVector<T> type that can easily be extended.
For example, right now MatrixSlice doesn't even implement the Vector interface.

So,
WeightedVector -> DecoratedVector<Pair<Integer, Double>>
MatrixSlice -> DecoratedVector<Integer>
NamedVector -> DecoratedVector<String>

We could even keep the names (maybe changing MatrixSlice to something like IndexedVector though?) by extending DecoratedVector<T>.

I'd be willing to fix this if people think it's a good idea.
What about it?