3D slicing with same shape: offset is off
From https://github.com/mratsim/Arraymancer/pull/306#issuecomment-425759168

```
# gflowback
gFlowback shape: [1, 10, 100]
gFlowback strides: [1000, 100, 1]
gFlowback offset: 0
gFlowback data length: 1000

# gFlowBack[timestep, _, _]
gFlowBack[timestep, _, _] shape: [1, 10, 100]
gFlowBack[timestep, _, _] strides: [1000, 100, 1]
gFlowBack[timestep, _, _] offset: 2000
gFlowBack[timestep, _, _] data length: 1000

# gflowback_ts = gFlowBack[timestep, _, _].squeeze(0)
gFlowback_ts shape: [10, 100]
gFlowback_ts strides: [100, 1]
gFlowback_ts offset: 2000
gFlowback_ts data length: 1000
```

Minimal repro:
```
import
  ../src/arraymancer

let x = zeros[int]([1, 2, 3])

let y = x[1, _, _]

echo x.offset # 0
echo y.offset # 6
```