Rework of Grackle

Rework of Grackle with version change (v2 to v3).

Now the code is a lot cleaner. I think if you wish to give a first feedback, it is a good moment.

The code currently works only with the mode 0 (interpolation from a table with a very basic chemistry). In comments, there is already a beginning of the future implementation for mode 1,2 and 3.

My next steps will be to implement mode > 0 and add some parameters in the yaml file. Do you wish to wait on theses for the merge?

Reproduction of Figure 2 in https://arxiv.org/abs/1610.09591.

Reproduction of Figure 8.

I am still getting some differences with the article, but I believe it comes from a different mode.

This graph is done with the CoolingBox test (the analytical solution is for a constant lambda cooling).