Add a more generic way of dealing with named columns for subgrid models

doc/RTD/source/ParameterFiles/parameter_description.rst

doc/RTD/source/Snapshots/index.rst

src/chemistry/EAGLE/chemistry_io.h

src/chemistry/GEAR/chemistry_io.h

src/chemistry/QLA/chemistry.h

src/chemistry/QLA/chemistry_io.h

src/chemistry/none/chemistry_io.h

src/parallel_io.c

src/part_type.c

src/serial_io.c

src/single_io.c

src/tracers/none/tracers_io.h