Small improvements to quickstart example
Improvements to docs
Change rest_frequency to rest_value
Don’t show stokes section until it’s written
Added spectral docs section
Allow string input for velocity convention
Remove rest_value from closest_spectral_channel and spectral_slab
Combine subsets.rst and spectral.rst into docs about modifying/manipulating cubes.
Re-order docs about accessing data and masking.
Fix small mistakes in docs
Mention what happens if a slice is not given for the data.
Update docs on extracting spectral slab
Added note about stokes
Implemented wording comments by @keflavich
Merge pull request #95 from astrofrog/update-docs

Update docs