Multi-channel emulators

Make the emulator controller support more than one channels of emulated front ends.

EmuTxCore and EmuRxCore now each contain a std::map of data/command buffers associated with a std::map for indicating whether or not a tx/rx channel is enabled. For now data buffers from all rx channels are aggregated in EmuRxCore when reading data, and they are read out one by one. Potentially they can be kept separate and fed directly to the corresponding data processors?

Also included are tests for running star emulators in multiple channels.