Only include non-empty cells in download package outputs

[cjgrady]

We are currently returning all cells in the geojson outputs but the cells without data are not really useful. Filter those cells out to reduce the size of the response and avoid errors caused by having too many cell values in the output package pages.

[cjgrady]

GitHub won't let me attach a test file (maybe it is too big?) But you can see the problem when there are too many sites in the "package/sitesObserved.js" file in the download package. I am not sure what that number is but it seems to work fine with 50k sites and not when there are more than 250k sites