Attaching provenance to multiple cells at once

[tkw1536]

Provenance should be attached to a cell range, not a single cell, i.e., to a (set-of-properties x set-of-items). The provenance info of a cell is given by all ranges covering it.

[tkw1536]

This was raised by @florian-rabe after the seminar. The current data model infact does explicitly assign one provenance to one cell using a ForeignKey. Ranges are thus implicitly supported, however multiple provenances for a single cell cannot be handled at the moment.

Furthermore, the importer currently creates one provenance for a cells of a single import.