switch proj-matrix caching off in calculate_attenuation_coefficients

It doesn't make sense to cache the projection matrix for this one-off calculation. This would be easy to do along the same lines as

STIR/src/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndProjData.cxx

Lines 676 to 682 in 59c3896

    
           if (auto sens_bp_pm_sptr = std::dynamic_pointer_cast<BackProjectorByBinUsingProjMatrixByBin>(this->sens_backprojector_sptr)) 
        
             { 
        
               // There is no point caching the projection matrix as we will use it only once 
        
               // Furthermore, disabling the cache will mean less memory used 
        
               // (and we don't have to release it) 
        
               sens_bp_pm_sptr->get_proj_matrix_sptr()->enable_cache(false); 
        
             }

	if (auto sens_bp_pm_sptr = std::dynamic_pointer_cast<BackProjectorByBinUsingProjMatrixByBin>(this->sens_backprojector_sptr))
	{
	// There is no point caching the projection matrix as we will use it only once
	// Furthermore, disabling the cache will mean less memory used
	// (and we don't have to release it)
	sens_bp_pm_sptr->get_proj_matrix_sptr()->enable_cache(false);
	}