[yt-dev] [yt_analysis/yt] Hardcoded values are used in several places in the code (issue #557)

[Kacper Kowalik]

New issue 557: Hardcoded values are used in several places in the code
https://bitbucket.org/yt_analysis/yt/issue/557/hardcoded-values-are-used-in-several

Kacper Kowalik:

Several physical constants are defined across multiple places in the code. As an example:

```
#!bash

$ grep '1\.98' -r
analysis_modules/halo_finding/halo_objects.py:        #Jc = 1.98892e33/pf['mpchcm']*1e5
analysis_modules/halo_profiler/multi_halo_profiler.py:        Msun2g = 1.989e33
utilities/units.py:    "Msun": (1.98892e33, mass),
utilities/tests/test_units.py:    msun_cgs = 1.98892e33
utilities/tests/test_units.py:    Msun_cgs = 1.98892e33
utilities/physical_constants.py:mass_sun_cgs = 1.98841586e33  # g
frontends/nyx/fields.py:    return (1/1.989e33)
frontends/nyx/data_structures.py:        self.units["particle_mass"] = 1.989e33
frontends/flash/fields.py:    return 1.0/1.989e33
frontends/art/data_structures.py:        cf["Mass"] = aM0 * 1.98892e33
frontends/castro/fields.py:    return 1.0 / 1.989e33
frontends/castro/data_structures.py:            self.units['particle_mass'] = 1.989e33  ### TODO: Make a global solar mass def
frontends/enzo/fields.py:    return data.convert("Density")*((data.convert("cm")**3.0)/1.989e33)
frontends/ramses/fields.py:    return data.convert("Density")*((data.convert("cm")**3.0)/1.989e33)
data_objects/universal_fields.py:    (3/(4*3.1415926535897931))**(0.5) / 1.989e33
data_objects/derived_quantities.py:    spin = J * E / (M*1.989e33*G)

```

Instead constants should be imported from `yt.physical_constants` everywhere for the sake of consistency.

Responsible: xarthisius