allow removal of HT bins from likelihood model  

Should be considered with pull request for suppress_low_ht_bin branch in ra1stats repo.

I've run limits with signal and the output seems reasonable. However, when removing HT bins, the ./test.py script outputs the usual fit result table to stdout at the end, which shows very large chi2 values. Not sure if this is a problem with the fit or just a "printing" issue. 

Not sure if the hack is correct. Needs checking.