EVS Multi-channel configurations: Add option to code LFE with 9.6 kbit/s NB EVS

For the multi-channel EVS reference conditions, the IVAS-3 Performance requirements mandates the following

For the EVS multi-mono system, the LFE is considered to be encoded with EVS 9.6 kbit/s NB.

From my understanding, the current scripts support the assignment of 9.6 kbit/s for the LFE channel, but it's not possible to limit the EVS bandwidth for this channel only to NB.