WS-2538 Fallback calibration PPR for restore delivery

I was informed that a CASA generated cal PPR with the naming scheme *pprequest.xml may exist in rawdata/ and should be used as the calibration PPR if a calibration doesn't contain a PPR.xml file. An example of this file is member.uid___A001_X360c_X68.hifa_calimage_selfcal.pprequest.xml.

This MR renames a calibration's *pprequest.xml file as PPR_calibration.xml in the products/ directory if it exists and the calibration didn't have a PPR.xml file.

I'm still scrounging to find a test for this, I haven't come across an ALMA cal with a pprequest.xml file yet.