Debug output of `SMatrix` is confusing

Consider the following code:

```rust
let matrix = Matrix2::new(1.0, 2.0, 3.0, 4.0);
println!("{:?}", matrix);
```
The output is:
```text
Matrix { data: [[1.0, 3.0], [2.0, 4.0]] }
```
As a casual user, just looking at the `Debug` output I'm likely to think that the matrix stored here is
```
[ 1 3 ]
[ 2 4 ]
```
whereas this is the transposed of the true matrix. The reason here is of course that `nalgebra` stores its matrices in a column-major format. However, perhaps we do not need to expose this fact in the `Debug` implementation. An idea here could be to use the same format as our new `matrix!` macro. In that case, the one-line output of `Debug` for the above matrix would read:
```text
[1.0, 2.0; 3.0, 4.0]
```
whereas with a pretty-print specifier (`"{:#?}"`) we could print it formatted with line breaks:
```text
[1.0, 2.0;
3.0, 4.0]
```

This issue led to some considerable confusion lately in #893 as a user wanted to port some code from `numpy`, and looking at the debug output it appeared that the output matrices were transposed.