Allow entry of keg volumes by weight

[danb35]

As mentioned at https://www.homebrewtalk.com/threads/keg-cop-keg-monitoring-and-control.692880/post-9308518, I think it'd be helpful for Keg Cop to allow for entering the quantity of beer in a keg using weight. Given empty weight of the keg, full weight (preferably allowing for units different than the system default for either--for example, I'd be using US gallons for my system, but the math is simpler with kilograms, as is parsing the input values), and specific gravity of its contents, the quantity of liquid can be easily and precisely determined.

[lbussy]

See if I can loop in additional information re: carbonation volumes according to this thread:

https://www.homebrewtalk.com/threads/keg-volume-calculator.633022/