removed category and added description to allergens.  The only categories we will need to know for the allergens are what patient group they are attached to.  A description could be useful for dietitians and users to be able to access
added unitwise gem for measurement units (tsp, lb, oz, etc)  and their conversions
adjusted ingredients_recipes attributes to match with unitwise formatting
left a placeholder method for converting formats but need to know which formats we will allow users to choose from and convert between
added attributes model