[fmatrix] add custom invert function 

- mainly copies the invert function from dune-common, except that the
LU decomposition always performs pivoting, not only if the diagonal entry
is below a threshold. This fixes the matrix inversion test for FieldMatrix
in dune-xt-la.

[cmake] add a 'license' target

which executes pylicense with all configs matching
"PROJECT_ROOT/.pylicense*.cfg"

[config.h.cmake] silence warning if FASP is not available

This reverts commit 2621d88d.
because cmake doesn't actually accept this despite what
the docs say

dailywork-ftschindler

dailywork-ftschindler

This reverts commit 1b6cfcb6.

dailywork-ftschindler