I have been called many things in my time, but a benchmark for human happiness is not one of them. Still, that was the job on offer when Fiat invited me to climb into the new electric Grande Panda and have my face watched by a laptop. Not in the slightly troubling way that some modern cars now seem to monitor whether you’re paying attention, but in the name of science. Or, at the very least, in the name of finding out whether a small, brightly coloured Fiat could cheer up a middle-aged car journalist. Fiat calls the idea smiles per hour, or SPH. The thinking is simple: car makers have spent years telling us about 0-62mph times, top speeds, power outputs and other pub-ammo numbers, but most people do not buy a car because it can shave a few tenths off a sprint time. Fiat reckons there is another number worth talking about: how often a car makes you smile.…