Add example import in ImpData

We have added an example import inside Program.cs. It is commented but
it provides a real use case of CsvImporter.

* Make _id_progetto public
* Move import logic so that each model is responsible for itself
* Use API_MOB_CLI_DOC_RIGHE instead of IGM_IMP_CLIFOR_RIGHDOC
* Add commented import example in ImpData
* Add commented diff stats in RowDiffImportStrategy