Implement specialized version of invert for 3x3 dense matrices.

We have used this kind of specialization in a down stream module to increase the performance of the ILU setup a bit

added backport label

added 89 commits

• d557c7fa...d1d354fb - 87 commits from branch master
• c8365a01 - [cleanup] Remove deactivate FMatrix inversion code
• ebbcd7ca - Add specialized code for inverting 3x3 dense matrix.

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merged

mentioned in commit b824570f

mentioned in merge request dune-geometry!4

mentioned in commit d29f78bf

mentioned in merge request !231 (merged)

mentioned in issue flyspray/FS#1712 (closed)

mentioned in issue #66 (closed)

mentioned in commit 691edc8c

mentioned in merge request !265 (closed)

mentioned in commit b61ff882