Merge branch 'master' into explict_bkg_cdim

...rTests/AdvectionDifferentTimeStepSizes_1D/plotSolution.py

...sts/AdvectionDifferentTimeStepSizes_1D/rt_advection1D.yml

...veTransferTests/AdvectionDifferentTimeStepSizes_1D/run.sh

examples/RadiativeTransferTests/Advection_2D/makeIC.py

examples/RadiativeTransferTests/CollidingBeams_1D/README

examples/RadiativeTransferTests/CollidingBeams_1D/makeIC.py

.../RadiativeTransferTests/CollidingBeams_1D/plotEnergies.py

.../RadiativeTransferTests/CollidingBeams_1D/plotSolution.py

...diativeTransferTests/CollidingBeams_1D/rt_collision1D.yml

examples/RadiativeTransferTests/CollidingBeams_1D/run.sh

examples/RadiativeTransferTests/CollidingBeams_2D/README

...ples/RadiativeTransferTests/CollidingBeams_2D/getGlass.sh

examples/RadiativeTransferTests/CollidingBeams_2D/makeIC.py

.../RadiativeTransferTests/CollidingBeams_2D/plotEnergies.py

.../RadiativeTransferTests/CollidingBeams_2D/plotSolution.py

...diativeTransferTests/CollidingBeams_2D/rt_collision2D.yml

examples/RadiativeTransferTests/CollidingBeams_2D/run.sh

src/cell.h

src/cell_unskip.c

src/cooling/const_lambda/cooling.h