Allow Prawnblaster to be triggered directly by a Pseudoclock

[dihm]

Given labscript limits the number of layers of PseudoclockDevices to 2, it is not possible to connect a PrawnBlaster to a PrawnDO (since the PrawnDO must be clocked by a PseudoclockDevice itself). It would be helpful if Prawnblasters could be triggered directly from a pseudoclock (from another Prawnblaster) instead of requiring an IntermediateDevice with Digital Outputs in between, in the same way PrawnDOs work now. Then one could fanout prawnblasters and PrawnDOs from a single parent Prawnblaster to get a large number of clock and digital output channels.