nm3networksimulator: Add acoustic receiver lockout #1
Description
When a physical modem is receiving an acoustic packet the receiver is locked out to any other acoustic packets arriving during this time.
The serial port on a physical modem is also non-functioning at this time to user commands so all input will be ignored and discarded.
If another acoustic packet arrives during lockout it will potentially corrupt all or part of the acoustic packet currently being demodulated. For now the simulator should discard both acoustic packets. Future refinements can calculate estimated probability of packet loss in this scenario.