Updates to IC generation routines in utils/ICs

[The9Cat]

Summary of changes

• Added a new routine, addring to generate a phase-space torus of material with a Gaussian velocity dispersion given a spherical halo model
• Fixed an error in the AxiSymModel:gen_point routine for particles at a given radius. This was not previously used by gensph.
• Updated AxiSymModel::gen_point routines to support new quiet-start methods implemented in gensph. These include distributing particles on a spherical position 'grid', distributing particles on a spherical position and velocity grid, Sellwood's quiet-start orbital plane algorithm, the same algorithm where orbital planes are simultaneously distributed on the sphere using a Fibonacci sequence
• Updated internal code in the model realization routines to use const values for algorithm constants rather than preprocessor values.

Comments

None of these changes affect existing EXP or pyEXP behavior. Perhaps addring might be dropped from this PR as too specific and idiosyncratic?

[michael-petersen]

All changes look good here -- but do we want to wait and do all IC updates simultaneously? Or we can merge this and start a new PR.

[The9Cat]

Maybe it's cleaner to add the gendisk quiet-start changes to this PR? The only advantage to merging now is the availability of the gensph quieter start. So do you want/need that any time soon? These things are rather specialized. E.g. the IC relaxation typically used for disk + halo will wipe out the quiet-start advantage.