Fg calc

[jpalmieri]

Calculation for Final Gravity estimate (the amount of unfermented sugars left in the beer after it's fully fermented, i.e., the yeast have processed all of the sugars that they can).

I removed the bit from the yeast model that was converting the form percentage (e.g., "75%") to a decimal percentage (e.g., "0.75"), as the conversion between the form and model was causing issues in recipe#edit and #show. I think it's cleaner to just convert it in the recipe model before it's used for calculations.

[eliotsykes]

Ok to merge, up to you if you want to make further changes on this branch or save for a later iteration/branch