Rework of Grackle

See merge request !484

Re-introduce the 'end_force' function as a task in order for the cooling function to be called at the right place in the time-step.

Align the runner structures on a memory bound.

Closes #395

See merge request !481

We might hope to be clean against all the issues it identifies

Comments mode > 0, initialize a_value, removed grackle_data from printing function (use cooling->chemistry now)

Fix the logging of neighbour ids in doself_subset_vec. Pull ids from main cell…

See merge request !479

Fix the logging of neighbour ids in doself_subset_vec. Pull ids from main cell instead of subset parts array.