FMatrixHelp::invertMatrix never used and DenseMatrix::invert not performant for blocksize>2
There is a method FMatrixHelp::invertMatrix that was used to invert a matrix before DenseMatrix was implemented. Nowadays the helper functions are still there in fmatrix.h. A duplicate is in densematrix.hh, but deactivated by
#if 0 ... #endif.
DenseMatrix::invert() has inversion code, but somehow the maple generated code is not used for dense matrices of size 3x3.
We should clean this up and add the 3x3 version again.