add support for computing the rank of matrices (!30)

this MR adds
- a new `rank` implementation to `Matrix`
- some tests


> **Note**  
> the algorithm is basically the same as in the matrix inversion from `invert`, i.e. transform the matrix into _echelon_ form and then count the number of non-zero rows

> **Note**  
> the row-rank of a matrix is the same as its column-rank, so we can safely count the number of rows