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
Status Job ID Name Coverage
  Test
passed #233270
test_min

00:03:13

passed #233271
test_mpi

00:04:43

failed #233272
test_mpi_fftw

00:05:13

failed #233273
test_mpi_fftw_hdf5

00:06:36

 
Name Stage Failure
failed
test_mpi_fftw_hdf5 Test There is an unknown failure, please try again
No job log
failed
test_mpi_fftw Test There is an unknown failure, please try again
No job log