DftNear2Far class contains no member function to compute Poynting flux of its near/monitor fields

[oskooi]

The DftNear2Far class which is instantiated using add_near2far contains a member function flux(self, direction, where, resolution) used to compute the Poynting of a "far" region specified by where. DftNear2Far contains no member function to compute the flux of its near/monitor fields.

(This is why the tutorial in #2629 involved two separate but overlapping DFT fields regions created using add_near2far and add_flux. The DftFlux class created using add_flux was used to compute the flux of the "near" fields.)