CDS units are not the same as regular units

[taldcroft]

For example:

In [5]: u.s is u.Unit('s')
Out[5]: True

In [6]: u.s is u.Unit('s', format='cds')
Out[6]: False

In [7]: u.s / u.s
Out[7]: Unit(dimensionless)

In [8]: u.s / u.Unit('s', format='cds')
Out[8]: Unit("s / s")

This first came up here: #7348 (review)

[kyleaoman]

Hm, this is even more complicated than it appears at first. Enabling cds units helps somewhat:

>>> import astropy.units as u
>>> u.Unit('s') is u.Unit('s', format='cds')
False
>>> import astropy.units.cds
>>> with astropy.units.cds.enable():
...     u.Unit('s') is u.Unit('s', format='cds')
... 
True

but not entirely:

>>> u.Unit('s') is u.s
True
>>> u.Unit('s', format='cds') is u.s
False
>>> with astropy.units.cds.enable():
...     u.Unit('s', format='cds') is u.s
... 
False

which implies the very confusing:

>>> u.Unit('s') is u.s
True
>>> with astropy.units.cds.enable():
...     u.Unit('s') is u.s
... 
False