MCB configuration on the GBT

The current GBT implementation of the SIB driver is accomplished on the main CPU board, in contrast to the use of a dedicated I/O processor on the VLBA systems. The redesigned MCB software will allow any later implementation of the MCB using different low-level hardware, without requiring modifications to the client programs. The redesigned software will have improved error checking, and will act to optimize the bus usage to the extent practical.