Implemented dict-like access for Stokes axes
Implemented dict-like access for Stokes axes

As per the second suggestion in #750 - the Stokes data can now be
accessed via cube['I'], cube['Q'] etc.

Assigning new keys is not yet supported and will throw
NotImplementedError if attempted.

keflavich/spectral-cube,radio-astro-tools/spectral-cube,e-koch/spectral-cube