fix for sTGC as-built correction normalization

reintroduce lenX in ElementModelSTGC and normalize as-built npar by lenX x lenY, which is 2 alpha h with measurement definition of alpha and h. This is a small bug-fix, necessary but not sufficient to make the sTGC as-built corrections work; at minimum the lenX and lenY parameters in the JSON file for the sTGC as-builts must be updated to the correct measurement-related values, but most likely some rotations by pi will eventually be needed to get all the signs right. If we are VERY lucky, further fixes can be done in the JSON.