A class used to store NMC values that are required further up the stack.  This class is immutable.
NMC values required by protocols higher up the stack are now stored in a NMC data object
down() method now returns the latest NMCData object when a HiTabEvent.GET_NMC_TIMES is received
The current synchronised clock time is now returned when PCSynch receives a HiTab.GET_CLOCK_TIME from a higher protocol