toch/np like indexig for DocumentArrayStacked #1073

da = DocumentArray([... for _ in range(5)]
da[0]  # index by offset; returns doc at this position
da[0:5:2]  # index by slice; returns every other doc
da[0, 1, 2]  # index by sequence of positions; returns DocumentArray with docs at these positions
da[[0, 1, 2]]  # same as above
da[[True, False, True, True, False]]  # index by boolean mask; returns DocumentArray with "True" docs
da[None]  # returns `da`, just to conform with PyTorch convention