Fixed: Deprecate WorkflowManager.export() in favour of output()

I've been meaning to do this forever. The day has finally come.

So now output() accepts the instructions that were previously served
by export(). But I kept export() for now; it'll be removed in VIS 2.
Docs: fixed rtype description for interval n-gram experiments in workflow.py

The documentation was incomplete because when an interval n-grams experiement
is run with "count frequency" set to False, the return type is a list of lists
of series but this was missing in the documentation.
Merge branch 'master' into devel_2