got rid of the need to create a whole new datacube to store the shifted ...

...data.  This should be more efficient in memory. 

Added in the option to return the arcsecond shifts required to coalign the data.  This is useful as a sanity check.  it is good to compare the shifts that were calculated against what you see in the final mapcube, and against what you thought expected.  Anyone have any better ideas about how to store/return this information?