small fixes

nifty/operators/dof_projection_operator.py

+import numpy as np
 from .linear_operator import LinearOperator
-from .. import Field
+from .. import Field, DomainTuple, dobj
 from ..spaces import DOFSpace
 
 

nifty/spaces/dof_space.py

 from .space import Space
-import numpy as np
 
 
 class DOFSpace(Space):
     def __init__(self, dof_weights):
         super(DOFSpace, self).__init__()
         self._dvol = tuple(dof_weights)
-        self._needed_for_hash +=['_dvol']
+        self._needed_for_hash += ['_dvol']
 
     @property
     def harmonic(self):