With a `.mesh` containing `Triangles` in a comment, like that one:

```
MeshVersionFormatted 1

Dimension
3

Vertices
121
-10.0402  -10.0402  -10.0402    0
-10  10  10    0
10  10  10    0
[...]
Triangles
609
     98     76     65    0
     98     81     76    0
     65     81     98    0
Tetrahedra
244
     81     65     76     98  0
     28      1     37     34  0
     69     25     70     94  0
[...]
End
```

then the code failed. It read `Triangles` as the beginning of the triangles section.

The commit use `std::getline()` to read the comments and the keywords:
- `Vertices`
- `Triangles`
- `Tetrahedra`