Error check qubits in QRegister

[ahelsing]

The QRegister constructor or perhaps the QGL2 compiler should error check that all qubits in the QRegister refer to existing named qubits in the channel library (accessed via the global static if necessary). However, we may need to consider the (future) usage where a QGL2 program is run WITHOUT first assigning to particular qubits, but rather the compiler is asked to do that assignment automatically.

See related tickets #31, #32