Allow completely empty input to be parsed correctly

Node
- default value for the lines array issued by prelude

Tests
- addition of a new test

Close #16
Implementation of a multiservice

Mode
- creation of the multiservice method
- conversion of the "validate" method to the new pattern

Code
- creation of a cache mechanism that prevents the execution of a service in case it has been executed on the same source code

Validation visitor
- code to perform the validation of the graph with the extraction of errors and warnings has been moved to this dedicated class

Partial fix of issue #50