Add more fields to type `Star`

[elehcim]

Temperature, Energy.

And for star particles: age, chemical composition (metallicity).

[islent]

I'm implementing a pure N-body gravity solver using PhysicalParticles.jl, so that even SPH info is redundant for now. SPH info is supported to suitably load Gadget2 snapshot files.

However, in the future (maybe distant future, since we are a small team), I have plans for MHD, numerical GR and cosmological simulations. In that case, many other fields are needed (for example, electric charge), and subgrid physics are supported and well-designed.

In brief, temperature, (total) energy, age, chemical composition (metallicity) will be supported very soon. However, this could never be an end before an adaptive and scalable subphysics data structure. (Need discussion! @longqian95 ).

[elehcim]

Thanks for the reply. I'll send you an email with my intentions.

[islent]

mry###1234@###mail.ustc###.edu.cn (# to be removed)
Welcome!

[elehcim]

Ok, for now the plan is to create a Gadget2Particle<:AbstractParticle3D in AstroIO to contain the fields needed by Gadget2 files.

I'll work on it.