Implementation of a multiservice for backend services

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
- handling of the case in which the source code is an empty string

Validation visitor
- code to perform the validation of the graph with the extractioon of errors and warnings has been moved to this dedicated class

Partial fix of issue #50