Replace PooledDataVector with PooledDataArray

PooledDataVector{T} is a typealias for PooledDataArray{T, 1}
PooledDataMatrix{T} is a typealias for PooledDataArray{T, 2}
PooledDataVector[NA, 1, 2] still exists
More ref() and assign() work is needed for PDA rank-n tensors