listmode objective function: actual_compute_objective_function_without_penalty (and Hessian) not yet implemented

@Imraj-Singh @gschramm sadly, we don't have it yet, unless @NikEfth has an implementation.

Without caching, it would essentially be a simplification of actual_compute_subset_gradient_without_penalty, with caching it's LM_distributable_computation

Pointers to Hessian functions:

STIR/src/include/stir/recon_buildblock/GeneralisedPrior.h

Line 84 in 8ced2d7

    
           virtual void accumulate_Hessian_times_input(DataT& output, const DataT& current_estimate, const DataT& input) const;

STIR/src/include/stir/recon_buildblock/GeneralisedObjectiveFunction.h

Lines 206 to 209 in 8ced2d7

    
           Succeeded accumulate_sub_Hessian_times_input(TargetT& output, 
        
                                                        const TargetT& current_image_estimate, 
        
                                                        const TargetT& input, 
        
                                                        const int subset_num) const;

	Succeeded accumulate_sub_Hessian_times_input(TargetT& output,
	const TargetT& current_image_estimate,
	const TargetT& input,
	const int subset_num) const;