Incorrect Imperial BMI Calculation with anything but "lb" as weight unit

[Firedorange717]

I use lbs as a weight unit since I am in America, which means I had to change the "Unit for Weight" field in the settings. I was startled to see that the BMI was calculated as being class III Obese which is not right in the slightest. After looking around through the code the problem appears to be in src/modules/weight/mixinBmi.js, it is looking for the weight unit to be "lb" not lbs which unless someone weights one pound is not quite right. If it is not "lb" it trys to calculate the BMI in Metric units which is why it is so off.

Perhaps as a fix have the Unit for Weight field be a dropdown with two options one for metric and one for imperial, that way no invalid values get passed through.

Thanks! I love the app its super useful.