Add missing property decorator

I have no idea why minimization has been working in the first place. The bug was
found by trying to access WienerFilterEnergy(...).value.
4 jobs for bug_wfEnergy in 6 minutes and 36 seconds
Name Stage Failure
failed
test_mpi_fftw Test There is an unknown failure, please try again
No job log
failed
test_mpi_fftw_hdf5 Test There is an unknown failure, please try again
No job log