Assess handling of large cubes

[rosteen]

This is probably a task that should wait until some of the other spectral-cube transition work is done. Essentially, spectral-cube takes some care in handling large cubes, in terms of avoiding copying data and providing hints on how to use memory mapping (from fits files) to handle cubes that are too big to fit in memory. It's possible that specutils handling of large data is already fine, but it would be prudent to do a side-by-side test with spectral-cube of a large dataset and see if some effort to improve large dataset handling/port over some strategies from spectral-cube would be prudent.