Slice

[StijnWoestenborghs]

Adding the SLICE operator.

• [x] forward
• [ ] backward
• [ ] unittests

As it is right now the operator does only support slicing in one dimension at the same time though. And therefore it behaves much more like np.take (https://numpy.org/doc/stable/reference/generated/numpy.take.html) then actual slicing. We might want to change that and support slicing for mulitple dimensions at the same time. But I just now figured out a parallelized and vectorized version for 1 dim at the time.