Attachment normalization and bug fixes

Attachments are now saved as the base filename (the path to the file is no longer included). If you need to preserve paths you should zip your file(s) before attaching. There are some bug fixes around import/export to from CSV and Google Sheets. The Python code has been updated to use the new versions of export_csv, export_sheet, import_csv, import_sheet found in libdataset.go shared library.

Improvements to some warnings and error messages. Minor bug fixes.