new drizcr default; more careful clobbering

--drizcr 1 is now the default
 * looks like the archive flt files (at least from CADC) don't have 4096
 flags already populated in the DQ array.  Probably the MAST flt/flc files
 would have those CR flags already planted.
 * So we now do CR rejection by default in the single-visit drizzle stage.
  If the user starts with cosmic ray flags set, this can be skipped by
  setting --drizcr 0.   If you want more CR rejection at the multi-visit
  stage, then the user can set --drizcr 2.

--clobber is now more careful with first-stage flts and ref images
 * previously, with --clobber on in the first stage, the entire epoch dir
   would get wiped away.
   Now we only wipe out the individual flt files.
 * previously the refimage could be clobbered unintentionally if the user
   set both --clobber and --doall.
   Now we only clobber the WCS refimage if the user also provides at least
   one of --reffilter, --refepoch, --refvisit.