More flexible energy scaling function

Merge request to add the option for different energy scaling functions. Three options are now available, set by the parameter EAGLEFeedback:SNII_energy_fraction_function:

• EAGLE: as in the original EAGLE simulations. This gives iso-fth contours that are diagonal lines in the log-Z vs. log-n plane.
• Separable: a transition from fE_min to fE_max that is separable in density and metallicity. This gives "boxy" iso-fth contours which curve upwards relative to the EAGLE scaling.
• Independent: a model where the energy scalings with metallicity and density are fully independent of each other. This creates a "four-box" pattern, with constant f_th towards each of the corners of the log-Z/log-n diagram and smooth transitions between them. This model has an additional parameter, EAGLEFeedback:SNII_energy_fraction_delta_E_n, which sets the maximum energy difference between low and high density at fixed metallicity.

The change only applies to the EAGLE_thermal model, the EAGLE_kinetic model is not changed @jborrow, please also have a look and check whether you can spot any issues.

examples/EAGLE_ICs/EAGLE_25/eagle_25.yml

@@ -181,8 +181,10 @@ EAGLEFeedback:
   SNII_sampled_delay:                   1               # Sample the SNII lifetimes to do feedback.
   SNII_delta_T_K:                       3.16228e7       # Change in temperature to apply to the gas particle in a SNII thermal feedback event in Kelvin.
   SNII_energy_erg:                      1.0e51          # Energy of one SNII explosion in ergs.
+  SNII_energy_fraction_function:        Independent     # Type of functional form to use for scaling the energy fraction with density and metallicity ('EAGLE', 'Separable', or 'Independent').
   SNII_energy_fraction_min:             0.5             # Minimal fraction of energy applied in a SNII feedback event.
-  SNII_energy_fraction_max:             5.0             # Maximal fraction of energy applied in a SNII feedback event.
+  SNII_energy_fraction_max:             1.0             # Maximal fraction of energy applied in a SNII feedback event.
+  SNII_energy_fraction_delta_E_n:       6.0             # Maximal energy increase due to high density (only used if SNII_energy_fraction_function is 'Independent').
   SNII_energy_fraction_Z_0:             0.0012663729    # Pivot point for the metallicity dependance of the SNII energy fraction (metal mass fraction).
   SNII_energy_fraction_n_0_H_p_cm3:     1.4588          # Pivot point for the birth density dependance of the SNII energy fraction in cm^-3.
   SNII_energy_fraction_n_Z:             0.8686          # Power-law for the metallicity dependance of the SNII energy fraction.