Helpers layer removed.

The code from the Helpers layer has been moved into the services layer.
The parsing is now made using a single instance of the Parser layer.
Using the  WorldBank XML as input (the biggest we have), the memory
consumption has been reduced in abount 900mb.
Close #54

The refactor has resulted in a notable disminution in the execution
time and in a minor reduction in the memory consumption.
Merge branch 'enhancement-#54' into develop