Change order of spectrograph and arm in data model from e.g. 1r to e.g. r1

Currently the data model specifies that we include the spectrograph number and arm in filenames as e.g.

 "pfsArm-%06d-%1d%1s.fits" % (visit, spectrograph, arm)

Unfortunately, some outputs (e.g. calibrated exposures) are written as

 "%1s%1d" % (arm, spectrograph)

so something has to be changed. I'd like to change it from e.g. "1r" to e.g. "r1" as:

1. consistency is vital!
2. the chip name can now be used as a variable
3. it's consistent with the naming we used in SDSS
4. while this change means that filenames will sort differently, as the main sort key is usually the visit the 12 files associated with a visit will still be together, and we feel that this is acceptable.

We will also need to switch the last two digits in raw file names (e.g. PFSA files); Craig and I agree that this is worth it at this point in the project where he owns all the raw data.

N.b. This is our last opportunity to make this change; once we've started taking data at LAM we are frozen for the rest of the life of the instrument.