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This is a bit cumbersome, as it involves yet another abstract
intermediate type, but necessary due to language limitations.

Setting a bandwdith of the input matrix via

elpa%set("bandwidth",bw)

tells ELPA 2stage, that matrix is already a banded matrix
with bandwidth bw. Then some compute steps can be skipped

By mistake a procedure has been named
"invert_tridiagonal" but the correct name should be
"invert_triangular"