Modify levels in DecamMapper.paf

[hsinfang]

Changes to fix the problem of not looping over ccdnum

[hsinfang]

"ccd" and "side" are not valid data id keys by default. If they are wanted, I probably should also modify decamMapper to update the keys, similar to the bottom of https://dev.lsstcorp.org/trac/ticket/2859

How useful are ID keys "ccd" and "side" in practice? Also I haven't gone far enough to know whether the levels "tract" is useful either.

[PaulPrice]

"ccd" and "side" allowed you to specify, e.g., N4 as ccd=4 side=N. I don't think the CCDs had official integer labels back when I originally wrote obs_decam, but now that it does (http://www.ctio.noao.edu/noao/content/decam-what), they're probably not necessary.

[hsinfang]

If the ID keys "ccd" and "side" are not very useful now we have ccdnum, I am inclined to not add them to keyDict. One reason is that "ccd" seems to be the default CCD key and used in some default implementation (for example, _getCcdKeyVal() in CameraMapper). Having "ccd" in keyDict might make it more error-prone because an integer is returned instead of no such a key found? Or should I not be concerned about that?