Add support for Metabolic Equivalent of Task (MET) quantity

[baarkerlounger]

uom supports kilocalories as a unit of energy. Having MET defined as well would help with calculating kilocalories in exercise contexts in a type safe way.

[iliekturtles]

Looks like there are a couple possible definitions: W / h and BTU / (h * ft^2). Is one more common than the other? Any idea what the quantity for either would be named?

[baarkerlounger]

I think that should be W / kg but that's the one I've seen used more often, and I'd expect the quantity to be called metabolic equivalent of task but I'm not sure if both are needed how best to distinguish them. Perhaps met_per_mass and met_per_surface_area?