Fixed element for lower triangular part of BTDMatrix

[MakiRuse]

The method element of BTDMatrix returns 0 only for the upper trianguar part of the matrix but not for the lower part. This results in a wrong display of the matrix for debug and if access is done through the "element" method, the resulting data will be wrong as well.

I propose this quick fix to also ignore the lower triangular part of the matrix when outside of the tridiagonal blocks.

[with-all-tests]

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[fredroy]

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