Analyser: add support for unknown variables that have been marked as external

[agarny]

Say that you have a model with an unknown variable (e.g., it misses an initial variable for the model to make sense). To analyse such a model will rightly result in an error being generated.

However, the analyser should be fine with the model if we were to flag such a variable as being external while, at the moment, it will still complain about the variable being unknown.