Using prior versions of constants
*********************************

By default, `astropy.units` are initialized upon first import to use
the current versions of `astropy.constants`. For units to initialize
properly to a prior version of constants, the constants versions must
be set before the first import of `astropy.units` or `astropy.constants`.

This is accomplished using ScienceState classes in the top-level package.
Setting the prior versions at the start of a Python session will allow
consistent units, as follows:

>>> import astropy
>>> astropy.physical_constants.set('codata2010')  # doctest: +SKIP
<ScienceState physical_constants: 'codata2010'>
>>> astropy.astronomical_constants.set('iau2012')  # doctest: +SKIP
<ScienceState astronomical_constants: 'iau2012'>
>>> import astropy.units as u
>>> import astropy.constants as const
>>> (const.M_sun / u.M_sun).to(u.dimensionless_unscaled) - 1  # doctest: +SKIP
<Quantity 0.>
>>> const.M_sun  # doctest: +SKIP
  Name   = Solar mass
  Value  = 1.9891e+30
  Uncertainty  = 5e+25
  Unit  = kg
  Reference = Allen's Astrophysical Quantities 4th Ed.

If `astropy.units` has already been imported, a RuntimeError is raised.