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
nifty/sugar.py                                                                25     21      4      0    14%
nifty/version.py 1 0 0 0 100%
------------------------------------------------------------------------------------------------------------
TOTAL 4047 1315 1117 193 65%
----------------------------------------------------------------------
Ran 8184 tests in 75.127s

FAILED (errors=80)
ERROR: Job failed: exit code 1
failed
test_mpi_fftw Test There is an unknown failure, please try again
Field: DEBUG: distribution_strategy set to default!
MPIFFT: INFO: Repacking d2o to fftw distribution strategy
--------------------- >> end captured logging << ---------------------

----------------------------------------------------------------------
Ran 8184 tests in 65.990s

FAILED (SKIP=5, errors=80)
ERROR: Job failed: exit code 1