[Dune] [#778] MatrixHelper::detAAT is unstable

[Dune]

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FS#778 - MatrixHelper::detAAT is unstable
User who did this - Oliver Sander (sander)

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Hi Martin!
The element that belongs to the matrix is not an element that I do finite elements on.  All I need to do is integrate over it.  Hence the extreme ill-conditioning is not a problem, and I don't even have to invert the matrix.  If the computed determinant was zero or nearly zero that would be fine.  I don't care much about precision here, I just cannot have the code abort.

Regarding your questions:
a) you can have separate algorithms for quadratic and non-quadratic matrices. Actually I wonder why you don't have those anyways.  Quadratic matrices are a common case and a dedicated algorithm for them may be a lot faster
b) It would really be a pity if we couldn't use generic geometries in dune-grid-glue (which is what I am talking about). They are perfect for the job.

I would much like to avoid the use of higher-precision types, because those extremely degenerate intersections really make up only a very small fraction.  In the vast majority of cases everything works fine with double precision.
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