[BUG] [Documentation] Multiple Subject Spreadsheet

[bendhouseart]

It's not clear to the user how to to use a multiple subject spreadsheet, see:

wasn't really able to understand how to use the multiple subjects spreadsheets: there seems to only be one input argument for the metadata, but there are two metadata sheets.

The 0th step would be to add a functional test to make sure the multi subject spreadsheet feature is working using the existing data in the many subjects folder.

[CPernet]

1 input for scanner -- always the same for all subjects
1 input for subjects -- the table has values that changes for each subject
output combines the fixed values (scanner) with subjects valus and output tsv/json for each subject
-- the name multisubject spreadsheet can be confusing -- the details is in the spreadsheet with a 's' is one spreadsheet for plenty of subjects