Fix mistake in THcShowerArray and make the threshold a parameter

THcShowerArray
1) in ProcessHits methodfFixed mistake in index of fThresh
   was using padnum instead of padnum-1
2) add parameter pcal_arr_AdcThreshold so that threshold
   can be set by a parameter. Default is zero which gives
   the best resolution.

THcShowerPlane
1) Added the parameter cal_AdcNegThreshold and cal_AdcPosThreshold
   so that the threshold can be set through a parameter
   Previously it was hardcoded.