Export file improvements

[enjalot]

A couple of things could make the export file format a little better:

1. include tags
   It probably makes sense to include a column per tag. Currently tags are stored as a file per tag which is just a list of indices into the original input data

2. make the combination of scope + input on-demand in the export page
   It's not clear if its a good idea to always regenerate the combined scope + input. I've found it convenient so far but if the input data is large it might add unneseccary slowness (especially if you only need the scope file downstream).