--gc:orc -d:openmp cause a segfault when calling sum(axis = 2)
```nim
import arraymancer

proc okproc*(tensor1: Tensor[float]) =
  var tmp = tensor1.sum(axis = 2).squeeze
  echo tmp

proc main() =
  let ex1 = ones[float]([20, 20, 8])
  okproc(ex1)

main()
```

Compiled with ``nim cpp --gc:orc -d:openmp`` produce a segfault.

Changin Tensor definition to : 
```nim
(...)
  storage* {.cursor.}: CpuStorage[T]
  ```
  seem to fix the issue.
  
  Alternatively dropping the ref in favor of ``SharedPtr`` (from ``fusion/smartptrs``) may be a solution as well.