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.