Creating band matrices

When working with stencil operations, one generally needs to work with band matrices.
These consists of diagonal entries + n neighbouring diagonals.

Example for a simple triband matrix:

```
2 1 0 0
1 2 1 0
0 1 2 1
0 0 1 2
```

I currently don't see a way to conveniently create such matrices.

It would be nice to have a function such as `from_bands`, which takes an uneven amount of vectors to create such matrices.

Example for the above matrix

```
Matrix4x4::from_bands( vec![1.0; 3], vec![2.0; 4], vec! [1.0; 3])
```