Fixed lb to kg / kg to lb conversion error.

[jonsagara]

For a weight of 175 lbs, I was expecting to see about 79.4 kg, but it was displaying 385.8.

Looking at the Weight class, it appears the conversions were backward:

kg = pounds * PoundsPerKilogram
lb = kilograms / PoundsPerKilogram

I believe it should be the other way around, so I swapped them:

kg = pounds / PoundsPerKilogram
lb = kilograms * PoundsPerKilogram

I also added a couple of unit tests to check the conversion.

Thank you,

Jon