tries to address #5 

## changelog
- the `rank.rs` example now only returns the _rank_ to `stdout`
- two tests have been added
  - one that asserts $\text{r}(M^T) = \text{r}(M)$
  - one that asserts $\text{r}(M) \leq \min(n, m)$ when $n$ and $m$ are the heigth and the width of $M$ respectively
- `Matrix::rank` has been rewritten thanks to ChatGPT, the full prompt can be found in the body of 384d9559 and the algorithm has been refactored and simplified in the following few commits

> **Note**  
> the two new tests do not pass on e27561bb but do pass on the tip of this MR branch, which indicates that the bugs have been solved