Returning shared_ptr<T> for methods marked const

Some methods are marked const, implying you can't modify the object. However, in a few places we return a non-const shared_ptr, which means the returned object can then be modified, and the original const object, too. If returning a class member (e.g., get_scanner_sptr), we should be returning shared_ptr<const T> if the method is marked const.

Follows from #480, #481 and #487. (KT edited after #481 updates)

List of problematic methods:

src/include/stir/ProjDataInfo.h:  inline shared_ptr<Scanner> get_scanner_sptr() const;
src/include/stir/RelatedViewgrams.h:  inline shared_ptr<DataSymmetriesForViewSegmentNumbers> get_symmetries_sptr() const;
src/include/stir/recon_buildblock/ProjMatrixByBin.h:  inline const shared_ptr<DataSymmetriesForBins> get_symmetries_sptr() const;
 
src/include/stir_experimental/recon_buildblock/ParametricQuadraticPrior.h:  shared_ptr<TargetT > get_kappa_sptr() const;
 
src/include/stir/recon_buildblock/BinNormalisation.h:             shared_ptr<DataSymmetriesForViewSegmentNumbers> = shared_ptr<DataSymmetriesForViewSegmentNumbers>()) const;
src/include/stir/recon_buildblock/BinNormalisation.h:            shared_ptr<DataSymmetriesForViewSegmentNumbers> = shared_ptr<DataSymmetriesForViewSegmentNumbers>()) const; 
src/include/stir/recon_buildblock/ProjectorByBinPairUsingProjMatrixByBin.h:  shared_ptr<ProjMatrixByBin> get_proj_matrix_sptr() const;
 
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearKineticModelAndDynamicProjectionData.h:  const shared_ptr<DynamicProjData>& get_dyn_proj_data_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearKineticModelAndDynamicProjectionData.h:  const shared_ptr<DynamicProjData>& get_additive_dyn_proj_data_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearKineticModelAndDynamicProjectionData.h:  const shared_ptr<ProjectorByBinPair>& get_projector_pair_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearKineticModelAndDynamicProjectionData.h:  const shared_ptr<BinNormalisation>& get_normalisation_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndProjData.h:  const shared_ptr<ProjData>& get_proj_data_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndProjData.h:  const shared_ptr<ProjData>& get_additive_proj_data_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndProjData.h:  const shared_ptr<ProjectorByBinPair>& get_projector_pair_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndProjData.h:  const shared_ptr<BinNormalisation>& get_normalisation_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndGatedProjDataWithMotion.h:  const shared_ptr<GatedProjData>& get_gated_proj_data_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndGatedProjDataWithMotion.h:  const shared_ptr<GatedProjData>& get_additive_gated_proj_data_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndGatedProjDataWithMotion.h:  const shared_ptr<GatedProjData>& get_normalisation_gated_proj_data_sptr() const;
src/include/stir/recon_buildblock/PoissonLogLikelihoodWithLinearModelForMeanAndGatedProjDataWithMotion.h:  const shared_ptr<ProjectorByBinPair>& get_projector_pair_sptr() const;

Start by reviewing the listed declarations in the src/include/stir and src/include/stir_experimental headers, along with the related changes in issues #480, #481, and #487. Check each const accessor and complete the work when the listed methods no longer expose mutable shared objects through const instances, with the project still compiling successfully.