Initialisation from undeclared value should raise an error

[clinssen]

The following model currently passes the model validation in NESTML, although it should raise an error. Variables in the state block are only allowed to refer back to internals and parameters, internals are only allowed to refer back to parameters, and parameters can refer back to neither but need to be defined in a self-contained manner.

    parameters:
        bar real = foo

    state:
        foo real = 123