Automatically computing connections

[natema]

The branch dev_varcon proposes a variable_connections function which automatically computes connection among components, under the assumption that all variables have unique names across subsystems.
After polishing the function (i.e. adding a check regarding the above assumption), we can use the function in all models.

[natema]

The function has now been improved by @piluc and @aurorarossi and is currently contained in the Earth4All repo. We should move it here.