warn users of crab-dev

[belforte]

with ref to #5133 I'd do in two steps:

1. always print a gentle warning when using crab-dev
2. make a new version now to be put in crab-dev where there is more forceful message to "STOP" and remove it when it will be time to move this to production (alas it will need a new tag). So this is just a temporary thing to wean CMSSW_12 users out of the "use crab-dev" mantra of last few months

Step 1. can be done as simply as with this in crab.py (to be verified in actual deployment)

if 'dev' in __file__ :
   print ("beware!")