Wiener filter with units & small volume factor correction

Hi,

Torsten asked me to incorporate a small Wiener filter example incl. units. Therefore I created a small ResponseOperator (including. convolution, exposure and domain/target space). Numerical test showed me that the create_power_operator does not give a proper PowerOperator (at least it does not match the results of comparable one in NIFTy_1). However it is only due to the incorrect incorporation of volume factors in the DiagonalOperator. The bare keyword has to be set to True.