Add scrunching before downloading files

[DavidSmith]

Add a small box below the 'perform query' action that has

sum frequency channels together by [256] sum subints together by [2] create total intensity [X] (polarization sum)

If the scrunch textboxes are blank, no scrunching of that kind will be performed. pscrunch is just a checkbox.

Files added to the cart with scrunching will show it in the cart list with extensions to their filenames of the form

filename_f256t2p

where f256 is the frequency scrunching factor, t2 is the subint scrunching factor and p is for pscrunch.

when the user selects download, these files will be created in a temporary directory using psrsh and added to the tarball with the new filenames.