Two (small) issues with display of tensor
example ([playground](https://play.nim-lang.org/#ix=2SAK)):
```nim
import arraymancer, sequtils
echo toSeq(1..24).toTensor().reshape(2,3,4)
```
output:
```
Tensor[system.int] of shape [2, 3, 4]" on backend "Cpu"
| | 	1	2	3	4 | 	13	14	15	16|
| | 	5	6	7	8 | 	17	18	19	20|
| | 	9	10	11	12 | 	21	22	23	24|
```

The two issues are:

1. an additional `"` after tensor shape
2. and additional set of `|` in first columns

First one is a trivial fix ([source](https://github.com/mratsim/Arraymancer/blob/master/src/arraymancer/tensor/display.nim#L21)); the second one might be a little be more involved ([source](https://github.com/mratsim/Arraymancer/blob/9a575de9e56fe3f5c10a5315765f5ded0332c44b/src/arraymancer/tensor/private/p_display.nim#L84)), but I could try to produce a PR with a fix for both.

Also, not sure what should be the expected display of a 4d tensor, an [example](https://play.nim-lang.org/#ix=2SAT):
```nim
import arraymancer, sequtils
echo toSeq(1..16).toTensor().reshape(2,2,2,2)
```
output:
```
Tensor[system.int] of shape [2, 2, 2, 2]" on backend "Cpu"
|						|
|-	-	-	-	-	-	-|
| | 	1	2 | 	5	6|
| | 	3	4 | 	7	8|
|-	-	-	-	-	-	-|
| | 	9	10 | 	13	14|
| | 	11	12 | 	15	16|
```

I noticed these two issues while writing a [remake of the Tutorial](https://pietroppeter.github.io/nblog/drafts/arraymancer_tutorial.html) using a library I am developing: [nimib](https://github.com/pietroppeter/nimib).

I also noticed this very interesting issue #488 which is relevant to documentation and for which maybe nimib in the future (I am close to first release of 0.1) might be able to help with.

