Fixed a bug that caused reprojected dask arrays to not be computable due to a temporary directory being removed

[astrofrog]

@keflavich - this is a fix for the issue we were seeing. Note that we need to keep the temporary directory around for a while because the user might call compute multiple times etc. So I think we might just need to document how to clean up the temporary directory once it is no longer needed.

[keflavich]

Yes, that will certainly be important. Will it naturally get cleaned up if the cube is deleted? It's not obvious to me that it will.

[astrofrog]

@keflavich - this won't get cleaned up automatically except by the operating system using the regular temporary directory cleanup. So this is not a good solution for spectral-cube. However, it's a fix that at least allows smaller cubes to be reprojected, so I'll go ahead and merge and work on a better solution.