Add possibility to only add newly found properties and do not overwrite already existing ones

[timkolloch]

Sometimes the foods collected from Tandoor already have properties assigned and one does not want those properties to be overwritten. For that case a possibility that already present properties are not overwritten should be added.

Th behavior should be as follows:

• No properties are present => all properties retrieved from USDA should be assigned
• Some properties are already present => only apply the ones that are not already present at the food
• All properties are present => Do not do anything, do not even request data from the USDA database

[timkolloch]

Reopen because I made a mistake in the commit message which I only noticed after merging the PR. I reverted the changes.