Convert between mass and volume of a particular ingredient
This would be useful in cases such as flour being purchased by kilogram and used in a recipe by cups. Conversion ratio is needed to make the calculation.
The app will learn the conversion ratio for a particular ingredient by asking how to convert from a unit of mass to a unit of volume. It will then use the density value whenever conversion from mass to volume is needed for that ingredient.
Ready for testing from build #76 on Test Flight.