IIndexBuilder interface for BigDeedle. This directly implements operations that can
be implemented on virtual vectors (mostly merging, slicing) and for other operations, it calls
LinearIndexBuilder. The resulting
VectorConstruction corresponds to the addressing
scheme of the returned index (i.e. if we return virtual, we expect to build virtual vector; if
we materialize, the vector builder also has to materialize).
Signature: unit -> VirtualIndexBuilder
Returns an instance of the virtual index builder