Add verification of session connections consistency in configuration

[alessandrothea]

All DAQ applications network endpoints in DAQ systems are supposed to have one or more endpoints to establish connections. The presence of unmatched network endpoints in a system configuration could lead to warnings, error or, if not properly handled, to crashes and non-deterministic behaviour.
Unmatched or dangling endpoints can be detected with the following steps

1. Loop over applications in a session
2. Gather the input and output network endpoints for all modules
   • if the application is a SmartDaqApplication, generate the list of modules, and then gather
3. Match inputs and outputs across applications
4. List non-connected endpoints
5. Raise warning/errors appropriately

[eflumerf]

This can also be done using create_config_plot. We have a "validate.py", but I don't know if we can do the generate_modules step from Python...