fix: use absolute path to weight file for SigmaNet

The current path works when doing make test because in that case the executable is started inside Rec/Phys/FunctorCore/tests/qmtest.
But that's of course super fragile and when one launches the test manually e.g. by ./Rec/run gaudirun.py Rec/Phys/FunctorCore/tests/options/test_functors.py the test will crash because it can not find the file.

cc @lecarus as original author.