Fix error in write
Change clobber -> overwrite in write
Merge pull request #88 from keflavich/issue88

``with_spectral_unit`` does not change mask WCS
Fix failure in Projection.write
Small improvements to docs
Merge pull request #93 from astrofrog/fix-projection-write

Fix failure in Projection.write
Preserved requested spectral unit
Tidy up initialization of new cubes from modified version of existing cubes.
Set _spectral_unit only in with_spectral_unit
Minor fix
Pass new meta
Fix propagation of spectral unit
Merge pull request #92 from astrofrog/improve-toc

Small improvements to docs
Check that WCS auto-changes unit and that spectral_unit is needed
Merge pull request #91 from astrofrog/preserve-spectral-unit

Preserve spectral unit requested
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