updated makeCrabFiles.py

Before, this would hang if the global tag was not provided, because it tried (and failed) to do the lookup in DBS (which is deprecated).

Now it looks for GTAG on DAS (the new DBS), automatically checking prod/global, prod/phys01, prod/phys02, and prod/phys03.  If it doesn't get a meaningful response, it tries to extract the gtag from the name.  If that also fails, it requires the user to input the gtag.  Note that the new file (das_client.py) is required for this lookup to work.  

This seems to return "UNKNOWN" a lot of the time from DAS; I talked with one of the DAS creators (Valentin Kuznetsov) about this, he says that's just a matter of the data that's available with DAS, and not a bug on his part or mine.....