Don't warn if there is no beam

[astrofrog]

By default there is a warning if there is no beam information:

/Users/tom/Dropbox/Code/radio-astro-tools/spectral-cube/spectral_cube/cube_utils.py:260: UserWarning: Could not parse beam information from header.  Exception was: TypeError('No BMAJ found and does not appear to be a CASA/AIPS header.',)
  "  Exception was: {0}".format(ex.__repr__()))

I don't think this should be shown by default since it's ok for cubes to not have a beam.