Gear cosmological simulations

I am almost ready to merge GEAR. I still need to clean up a bit the code and to make the final plots.

This is a large merge request, but mostly on GEAR's files. It contains a lot of small fixes.

• I removed the task order for the cooling.
• I updated the ZoomIC example.
• I removed the example DwarfGalaxy.
• I added GEAR in doxygen (and fixed all the warnings)
• I implemented the named columns, but to do so, I had to update the API for chemistry_write_flavour.
• Implement #650 (closed).

My next steps will be (not necessarily in the order):

• Finish my work on the star formation splitting + compare with GEAR,
• Convince Yves about SPHENIX vs PU,
• Writing my thesis + SWIFT's documentation,
• More testing,
• Being more consistent between GRACKLE and the star formation (e.g. temperature)
• Trying to optimize the code.
• Start working on sink particles.

doc/Doxyfile.in

@@ -770,12 +770,18 @@ INPUT		       += @top_srcdir@/src/cooling/const_du
 INPUT		       += @top_srcdir@/src/cooling/const_lambda
 INPUT		       += @top_srcdir@/src/cooling/Compton
 INPUT		       += @top_srcdir@/src/cooling/EAGLE
+INPUT		       += @top_srcdir@/src/cooling/grackle
 INPUT		       += @top_srcdir@/src/chemistry/EAGLE
+INPUT		       += @top_srcdir@/src/chemistry/GEAR
 INPUT		       += @top_srcdir@/src/entropy_floor/EAGLE
+INPUT		       += @top_srcdir@/src/pressure_floor/GEAR
 INPUT		       += @top_srcdir@/src/star_formation/EAGLE
+INPUT		       += @top_srcdir@/src/star_formation/GEAR
 INPUT		       += @top_srcdir@/src/tracers/EAGLE
 INPUT		       += @top_srcdir@/src/stars/EAGLE
+INPUT		       += @top_srcdir@/src/stars/GEAR
 INPUT		       += @top_srcdir@/src/feedback/EAGLE
+INPUT		       += @top_srcdir@/src/feedback/GEAR
 INPUT		       += @top_srcdir@/src/black_holes/EAGLE
 INPUT		       += @top_srcdir@/logger