Speed improvement keeping files open

[HerManNav]

A posible improvement for performance could consist of keeping the file open until the user called a specific method to close the file when all the modifications have been performed. This method SHOULD be called also in the destructor of the dirB instance!

The main drawback of this approach is that the file wouldn't be accessible by other applicactions while the instance of the dirB lasted.

A third approach is to let the user select between these 2 modes (performance vs accessibility modes).