Schmidt law for the star formation rate

Adding the option to use a Schmidt law for star formation within the EAGLE star formation module.

The code defaults to the pressure law with no extra parameter necessary in the yaml file. In order to use the Schmidt law, add the first or both lines to the yaml file:

  SF_model:                     SchmidtLaw     # (Optional) If set to "SchmidtLaw", the SFR is calculated using a Schmidt law, defaults to the pressure law
  star_formation_efficiency:         0.01      # (Optional) If a Schmidt law is used, this regulates the star formation efficiency per free-fall time (defaults to 0.01)

Mostly copied over from @folkert's work for COLIBRE with smaller adaptations. I also made sure that the IsolatedGalaxy_feedback example runs out of the box again (download the new cooling tables and minor changes to the python script that tests the SFH). The condition for star formation remains untouched, the only part that is affected is the calculation of the star formation rate.

The code is compiling and running for both the EAGLE pressure law and the Schmidt law (tested with IsolatedGalaxy_feedback).

examples/EAGLE_ICs/EAGLE_100/eagle_100.yml

@@ -114,6 +114,7 @@ EAGLECooling:
 
 # EAGLE star formation parameters
 EAGLEStarFormation:
+  SF_model:                          PressureLaw
   EOS_density_norm_H_p_cm3:          0.1       # Physical density used for the normalisation of the EOS assumed for the star-forming gas in Hydrogen atoms per cm^3.
   EOS_temperature_norm_K:            8000      # Temperature om the polytropic EOS assumed for star-forming gas at the density normalisation in Kelvin.
   EOS_gamma_effective:               1.3333333 # Slope the of the polytropic EOS assumed for the star-forming gas.