Merge branch 'gear_metal_diffusion' into 'master'

configure.ac

examples/SubgridTests/SmoothedMetallicity/makeIC.py

examples/SubgridTests/SmoothedMetallicity/plotSolution.py

...SubgridTests/SmoothedMetallicity/smoothed_metallicity.yml

examples/parameter_example.yml

src/Makefile.am

src/chemistry.h

src/chemistry/EAGLE/chemistry.h

src/chemistry/EAGLE/chemistry_iact.h

src/chemistry/GEAR/chemistry.h

src/chemistry/GEAR/chemistry_iact.h

src/chemistry/GEAR/chemistry_io.h

src/chemistry/GEAR/chemistry_struct.h

src/chemistry/GEAR_DIFFUSION/chemistry.h

src/chemistry/GEAR_DIFFUSION/chemistry_iact.h

src/chemistry/GEAR_DIFFUSION/chemistry_io.h

src/chemistry/GEAR_DIFFUSION/chemistry_struct.h

src/chemistry/QLA/chemistry.h

src/chemistry/QLA/chemistry_iact.h

src/chemistry/none/chemistry.h