Suggestion: Open multiple sessions / clone multi-documents

[saarinel]

The following suggestion is related to (may overlap with) issue #119 and might be essentially the same proposition - thus I will pull it if requested:

Possibility to open multiple Jdemetra+ sessions at once or to clone multi-documents in Workspace – Seasonal adjustment – multi-documents. Intuition behind user feedback: This would ease analysis of changes in series pre- and post SA.