Create a test for reimport of transactional data

[dcolleoni]

Related to #139

Implement test for:
-reimport of opening
-reimport of actual
-reimport of cashflow

[sbusato]

Use case1:
Consider a data node at inception, create openings for this data node and import the file multiple times.
=> the result should not change.

Use case2:
Consider one data node at inception and one that is not. Create an Actuals file that contains data for both these two data nodes. Import this file multiple times.
=> the result should not change.

Use case3:
Consider one GRIC that is linked to one GIC. The GRIC already existed the previous year, while the GIC is at inception.
Import data for the GRIC in the previous period.
Import Openings for the GIC in the current period.
Import Cashflows for the GRIC in the current period.

@dcolleoni

[amuolo]

@dcolleoni @sbusato who is working on this ticket?