~vonfry/omnetpp-nix

Merge branch 'develop'
omnetpp: update python to python3
readme: update todo and add flake description
inet: update to 4.2.8
ops: update to 20211019
dev/nix: add flake
dev/nix: add flake
omnetpp: update to 5.7
omnetpp: update to 5.7
nix: stdenv.lib is deprecated.
omnetpp/keetchi: withDoc as false
fix omnetpp wrap, because the configure script dont read OMNETPP_IMAGE_PATH
revert patch files, which is easy
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