[common.fmatrix]
* provide empty ctor
* disable checks in NDEBUG environment
* specify Dune::FieldVector (in contrast to Stuff::Common::FieldVector)
[stuff.common.fvector] provide default ctor
[common.configtree] added support for stuff variants of fvector/fmatrix
[functions.spe10] added 2 -> 2x2 variant

This is just the unit matrix scaled by the Kx value, not the actual 2d
Spe10 data.
[grid.fakeentity] whitespce

The whitespace here was on purpose for reasons documented inside my head.