Added a new example: A small cosmological volume with some cooling on.

examples/SmallCosmoVolume/small_cosmo_volume.yml

examples/SmallCosmoVolume_cooling/README

examples/SmallCosmoVolume_cooling/getIC.sh

examples/SmallCosmoVolume_cooling/plotTempEvolution.py

examples/SmallCosmoVolume_cooling/run.sh

examples/SmallCosmoVolume_cooling/small_cosmo_volume.yml

src/cooling/const_lambda/cooling_io.h

src/parallel_io.c

src/serial_io.c

src/single_io.c