Correct bug #191: Non persistent data are never removed from the file system.

Correct bug #191 

I change the constructor of medNonPersistentDatabesImporter in order to always construct a base medAbstractDatabaseImporter with the indexWithoutImporting property set to true, so the images are not write in the database. 

As you can see, we still have some works to do in that area, for example there is a constructor that allow to create a medDatabaseImporter object with the indexWithoutImporting prpperty set to true as well, I can't see any reason one would want to do that.