ENH: use singleton method for accessing runtime selection

...RFoam/PDRModels/dragModels/PDRDragModel/PDRDragModelNew.C

...tion/PDRFoam/XiModels/XiEqModels/XiEqModel/XiEqModelNew.C

...bustion/PDRFoam/XiModels/XiGModels/XiGModel/XiGModelNew.C

.../solvers/combustion/PDRFoam/XiModels/XiModel/XiModelNew.C

...tyModels/mixtureViscosityModel/mixtureViscosityModelNew.C

...ocityModels/relativeVelocityModel/relativeVelocityModel.C

...DTRM/reflectionModel/reflectionModel/reflectionModelNew.C

...oPhaseMixtures/temperaturePhaseChangeTwoPhaseMixtureNew.C

...haseChangeTwoPhaseMixture/phaseChangeTwoPhaseMixtureNew.C

...cellSizeAndAlignmentControl/cellSizeAndAlignmentControl.C

...aces/cellSizeFunction/cellSizeFunction/cellSizeFunction.C

...ionType/cellSizeCalculationType/cellSizeCalculationType.C

...unction/surfaceCellSizeFunction/surfaceCellSizeFunction.C

...AreaWeightModel/faceAreaWeightModel/faceAreaWeightModel.C

...ialPointsMethod/initialPointsMethod/initialPointsMethod.C

...noiMesh/relaxationModel/relaxationModel/relaxationModel.C

...esh/searchableSurfaceFeatures/searchableSurfaceFeatures.C

.../manipulation/createBaffles/faceSelection/faceSelection.C

...s/foamHelp/helpTypes/helpBoundary/helpBoundaryTemplates.C

...s/miscellaneous/foamHelp/helpTypes/helpType/helpTypeNew.C