Integration of astropy into spectra

There are many flaws in this change. Please give me your reviews, comments to make this better! 
@kaichogami this starts to get tricky, because, in the case of Callisto (which is the only for what this was written for) there are not physical units, just counts or data number.
`u.counts` or `u.ct`.  However, STEREO/SWAVES must have some units... but 
[their](http://stereo-ssc.nascom.nasa.gov/data/ins_data/swaves/2012/swaves_average_20120704_a_lfr.dat) 
[files](http://stereo-ssc.nascom.nasa.gov/data/ins_data/swaves/2012/swaves_average_20120704_a_hfr.dat) 
are useless...  I think in part it must be due that the people don't really care in their units... but I will have to find out.  At least, CALLISTO header says 'digits'... 

Now, how do we translate that in a way that it makes sense when plotting?  I don't know.  I'm not sure if we want the people fight with such units... @Cadair, @ayshih any ideas? We will have similar problem with maps where different instruments may have different units.