Add file writing process to hxActors

When reading ramps we need to stay ahead of the DAQ. I currently read from the DAQ and write to the FITS file in the same process, but any delay causes the DAQ to run out of buffer and for the reads to fail. A recent change in the readout sequence to minimize the effect of some electronic noise is making us hit the hardware limits.

Add a process which handles the file writing. The tricky part is that we promise to generate keywords when we have flushed individual reads and the entire PFSA and PFSB files, and decoupling readouts from file writes will make that less trivial.