sensitivity calculation with ProjData can do unnecessary forward projections

We currently set the forward projector in the call

Line 887 in 208fd3c

    
           distributable_sensitivity_computation(this->projector_pair_ptr->get_forward_projector_sptr(),

which will do

Line 408 in 208fd3c

forward_projector_ptr->set_input(*input_image_ptr);

However, for Parallelproj and NiftyPET projectors, this means that the projection is done, even though it is never used.

There are 2 ways to prevent this:

pass a 0 pointer as forward projector and adapt distributable_computation
change Parallelproj forward projector to only do the projection if at least one projection of a related_viewgrams (or whatever) is called.

The first options is easiest...