[Type] Mat: add unit tests and fix isDiagonal()

[fredroy]

While adding some unit tests, I noticed that isDiagonal was wrong 😑
The problem occurs when i=0 (first row). The loop condition is while j<i-1 but when i=0 it will become j<std::max<sofa::Size> because i and j are unsigned.
The fix is also testing if the matrix is rectangular but not sure if we want that 🤔 (like for determinant?)

[with-all-tests]

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[alxbilger]

Wikipedia says:

The term diagonal matrix may sometimes refer to a rectangular diagonal matrix, which is an m-by-n matrix with all the entries not of the form di,i being zero. For example:

$$ \begin{bmatrix} 1&0&0\\ 0&4&0\\ 0&0&-3\\ 0&0&0\\ \end{bmatrix} \quad \text{or}\quad \begin{bmatrix} 1&0&0&0&0\\ 0&4&0&0&0\\ 0&0&-3&0&0 \end{bmatrix} $$

[fredroy]

[ci-build][with-all-tests]