add support for computing the rank of matrices

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

changed the description

added 1 commit

• a478f91c - allow `Matrix::rank` to be _dead code_

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merged

mentioned in commit 02ead962