Introduction of a Property.doi attribute

[Allentro]

I was interested in exploring the source papers for some of the data here.

I was wondering whether there is any interest in a 'doi' attribute to the property class?

I was just going to look into some of the source papers from the database and this seems like a simple and robust way which would compliment Property.source

[Allentro]

Just spotted this already exists further down in the property.py file.. It's an attribute of the bibsource