Change the name of the commands that export data

[braganholo]

Instead of now prov, now dataflow, use now export with five different options: prov, prolog, dataflow, ast, and jupyter notebook. Maybe we also need one to export diffs in jupyter notebooks (need to check).

This would imply to remove the "export" command that currently exports Prolog.

The description of the current export AST command seems to be wrong. Need to check and correct.