ZDCFitWrapper: Initialize constant term in PrePulse wrapper in DoInitialize()

Change from @bcole restores missing initializer in ZDCFitWrapper.cxx in ZDCFitExpFermiPrePulse::DoInitialize. I ran @christos setup from https://its.cern.ch/jira/browse/ATLASRECTS-7759 twice in two separate directories (just changing the raw data to a Minbias file from Run 461681). I also ran his "acmd diff-root" script (as given) on the two files and I see no change.

Py:diff-root         INFO comparing over [100] entries...
Py:diff-root         INFO comparing [1138] leaves over entries...
Py:diff-root         INFO Found [2204448] identical leaves
Py:diff-root         INFO Found [0] different leaves
Py:diff-root         INFO all good.

I would appreciate a cross check of this, but this looks promising. Scripts and output can be found here: /eos/atlas/atlascerngroupdisk/det-zdc/norepro/