Fix FIXME in tpx

[don la dieu]

tpx contains an old FIXME in a #warning. FIXME's need to be fixed, or turned into proper documentation as a code comment.

For context:

if (domStatus.domGainType == I3DOMStatus::High &&
     (!std::isfinite(vemCalibration.hglgCrossOver) || vemCalibration.hglgCrossOver<=0))
    {
 #warning FIXME: This is kind of fishy. Subsequent modules should not have to do this check again.
      // Only warn and use default value in case of problems. Not a critical error.
      log_warn("Invalid \"hglgCrossOver\" value (%.2f) for module %s. Using default value of 3000 pe!",
             vemCalibration.hglgCrossOver, omKey.str().c_str());
    }
 
  return true;
 }

[Meike de With]

This is a duplicate of #409