Optimise fancy indexing further

Continue to fine tune functionality and performance of fancy indexing according to outline suggested in #437

- [ ] work around for ndindex not supporting array indices separated by slices e.g. arr[0, :, [0, 1]]
- [ ] pure boolean indexing can definitely be optimised to work more like LazyExpr where for more general shapes (not just key.shape = array.shape)
- [ ] implement vindex (for numpy and blosc array return types)
- [ ] finish implementation of oindex (return blosc array)
- [ ] optimise fancy indexing for broadcasted index arrays
- [ ] implement (optimised) setitem for fancy indexing
- [ ] implement fancy indexing in slice (i.e. return blosc array not numpy array)
- [x] ~~optimise fancy indexing to avoid recalculating _slice/chunk_slice intersections every loop~~ (PR #459)