Property API fixes

[miratepuffin]

This is a master ticket to jot down annoying things with the property API's

• Edge property APIs are a bit of a hack under the hood - this means that that are a couple orders of magnitude slower than node properties
• If you ask for properties on edges/nodes and all of the entities do not have this property you will get a None, if some of them do you will get an iterator of Options - this means you have to check fi the returned object is None, instead of just being about to iterate over an empty iterator.
• If I call get on an individual Node/Edge I will get a None if that entity doesn't have that property (understandable), but I will also get Nones when calling on Edges/Nodes - this means that I can only call sum() etc when all Edges/Nodes have this property. As we know the type of the property (assuming at least one entity has it) it makes much more sense to return a default value i.e. 0, "", or []