Added PTS-via-PB as child of Slave PulseBlaster

Added device PTS-via-PB to implement control of a PTS-160 RF source over
the parallel entry card interfaced via a Slave Pulseblaster.  Wiring
assumes PB-PTS-Interface v1.1c and PTS-Amplitude v1.1c and compatible
circuit boards mediating the interface.

Works for at least the straight-through fast mode, testing on high
resolution mode is waiting for circuit hardware changes.