Add chemistry in part

@pdraper, with @matthieu, we have discussed about adding the cooling data to the part (e.g. density of H, He, ...).

Here is a quick version in order to discuss how to make it compatible between the grackle and eagle cooling.

I have written a way to:

• read a snapshot
• write a snapshot
• interact the chemistry of two particles during the density tasks (we can imagine doing the same during the force and ghost tasks)

Feel free to modify this branch if you have a suggestion.