Distinguish Zeros from import in Scenario and Zero due to missing Data

[calderinisystemorph]

In the implementation of Scenario Importers it was enabled to save zeros for RawVariable and IfrsVariable, due to fact that users may need to "suppress" some line in the Cashflow or Actuals. This feature led to an implementation which is not optimal. Hence, is important to distinguish between a compact, meaningful set of zeros that are representative of the business logic and discard the rest.

[dcolleoni]

@calderinisystemorph the idea here is to save only those 0s that lie in the intersection of the SCE and BE sets of IfrsVariables.
Thanks for including this in the open PR.
Davide

[amuolo]

I think we can close this ticket because it has been solved with the PR #112
@dcolleoni @calderinisystemorph