Star formation sub grid update

I changed the temperature criteria to an entropy criteria to be faster, furthermore I added the birth temperature to the snapshots and added a histogram plot to the solutions.

So I run tests using the lowres8 isolated galaxy without feedback, and I don't seem to see any difference in the results: Old: New:

Same for the SFH: Old: New:

I will run the one with feedback to check if it gives a similar result.

One thing, I assumed that in general we want to keep the functionality to call the cooling function in star_formation_is_star_forming function for other subgrid models, so that function still has the required arguments.