Add nuclear star cluster component

[liempi]

Add nuclear star cluster model and BH seed formation scenario within NSCs due to stellar collisions.

[abensonca]

Adding comments from Matias here for reference:

1. First, in the objects.nodes.components.black_hole.standard.F90, the Node_Component_Black_Hole_Standard_Scale_Set subroutine has 1 M⊙ for the mass scale, it is because otherwise it leads to negative masses. I think it would be nice to introduce a new relative mass scale with the stellar mass of the NSC, maybe when the NSC already formed a seed.
2. The BH seed formed under the model is created in the nodes.operators.physics.stellarCollisions.NSCs.F90 file. I tried to do move this node operator to the blackHoleSeedClass and it is not possible because as there are not BHs in all the galaxies, it ends in an error in routines using the potential in their calculations (as the mass of the BH is 0.0).
3. In the objects.nodes.components.black_hole.simple.F90, I'm not sure if to introduce a modification. In the rate task, in principle we should remove the gas accreted also for the hot halo as in the standard file but it is not implemented. If it makes sense, I can do it.

and not related to my projects, I think there is a mistake in the cooling.cooling_radius.beta_profile.F90 file. In line 381, the variable is called densityZero which is supposed to be the density evaluated at radius 0.0, but it is using the coordinates of the outerRadius of the Halo (previously used to compute the density at the outer radius).

[abensonca]

and not related to my projects, I think there is a mistake in the cooling.cooling_radius.beta_profile.F90 file. In line 381, the variable is called densityZero which is supposed to be the density evaluated at radius 0.0, but it is using the coordinates of the outerRadius of the Halo (previously used to compute the density at the outer radius).

Yes - definitely a mistake. I opened issue #781 for this and will push a fix.

[abensonca]

1. First, in the `objects.nodes.components.black_hole.standard.F90`, the `Node_Component_Black_Hole_Standard_Scale_Set` subroutine has 1 M⊙ for the mass scale, it is because otherwise it leads to negative masses. I think it would be nice to introduce a new relative mass scale with the stellar mass of the NSC, maybe when the NSC already formed a seed.

This seems reasonable. I'd suggest making it some fraction of the NSC stellar mass.

[abensonca]

2. The BH seed formed under the model is created in the `nodes.operators.physics.stellarCollisions.NSCs.F90` file. I tried to do move this node operator to the `blackHoleSeedClass` and it is not possible because as there are not BHs in all the galaxies, it ends in an error in routines using the potential in their calculations (as the mass of the BH is 0.0).

I caught this problem in the PR refactoring black hole physics, and it is now implemented correctly there. We'll need to be sure to move the equivalent NSC behavior to this new class (after I merge in that PR).

[abensonca]

2. The BH seed formed under the model is created in the `nodes.operators.physics.stellarCollisions.NSCs.F90` file. I tried to do move this node operator to the `blackHoleSeedClass` and it is not possible because as there are not BHs in all the galaxies, it ends in an error in routines using the potential in their calculations (as the mass of the BH is 0.0).

It would be better to move this to the blackHoleSeedClass if possible. If the only issue is the black hole potential, then we should change the massDistributionBlackHole class so that it correctly handles cases of zero mass black holes (just returning a zero potential in such cases). I'm actually surprised this happens as in the Node_Component_Black_Hole_Standard_Mass_Distribution function it checks that the black hole mass is $>0$ - and if it isn't returns a null massDistribution - in which case this problem should never occur. But maybe that's not working for some reason.