`@node` should not support interface names with underscores

[bvdmitri]

@ismailsenoz noticed that the following snippet fails to create a proper node:

@node SomeNode Stochastic [ out, some_interface ]

The problem is that interface names must not include _ in its names as it is reserved for joint marginals. This example must throw an error in advance such that it does not confuse a user (and us).