add_terms() replaces add_term() and add_free_terms()

For RapidPro import - new column 'network provider' on item

There are two new item types 'question' and 'rumor'.
Item types for GeoPoll data are 'question'.
Item types for RapidPro data are 'rumor'

It will now trim the columns to the number defined in the map.

ItemTransport clas --> transport.items module

Replace with ItemTransport object, which needs to be instantiated. I
think it can be reused between calls to its methods so there's no need
to create new ones all over the place, but that's what I have done so
far. I'm a bit uncertain about module level variables, and not
everything is contained in a class.

This also fixes error handling for malformed dates

It is now possible to specify a date format string (as used by
strptime) for date columns in the spreadsheet.

Still need to handle row numbers correctly

Includes a hacky fix to skip ignored values and a temporary hardcoding
of the sheet profile.

normalize_row doesn't really need to dealw ith None
and theres a slightly more elegant way to calculate row numbers

Google CSV to XLSX conversion results in dates being stored as date objects
rather than strings, so we need to cope with these