Added all the files required for the Compton cooling model.

configure.ac

doc/Doxyfile.in

src/Makefile.am

src/cooling.h

src/cooling/Compton/cooling.h

src/cooling/Compton/cooling_io.h

src/cooling/Compton/cooling_struct.h

src/cooling/const_du/cooling.h

src/cooling/const_du/cooling_io.h

src/cooling/const_du/cooling_struct.h

src/cooling/const_lambda/cooling.h

src/cooling/const_lambda/cooling_io.h

src/cooling/const_lambda/cooling_struct.h

src/cooling_io.h

src/cooling_struct.h