fixes #2

you can now chain multiple hook-scripts per
hook name
see hooks/multihooks.py docstring for detailed instructions

which is now the current one in our debian dockers

The 3.7 one was never updated to match the 3.8 one.

This reverts commit 3ef61814.

The original one was not the one from dune-xt-common

This reverts commit ebc4a4c9.

This is in line with the clang-format specs, otherwise we will loose the
2 empty lines around classes, etc...